Reference
Glossary
Plain-language definitions of the words this manual, the photo apps and the archives use.
Each term links the chapter that explains it. Product names (Optimize Storage, Storage saver, Takeout) are the makers' own words as of 2026-10-30.
| Term | Meaning |
|---|---|
| 3-2-1 | Three copies of your photos, on two kinds of storage, with one away from home. It began as a photographer's rule and is the advice the National Archives gives families. This manual's target; two copies in different places is the first milestone. See Backups. |
| Access key | The code Apple gives when you add a Legacy Contact. The contact needs it, with a death certificate, to request access to your account's data. See Routines and handing on. |
| Appraisal | An archivist's word for deciding what is worth keeping, by significance rather than money. This manual adapts it to a family library. See Choosing what to keep. |
| Archive | A set of files kept for the long term, as distinct from the copy you look at every day. In this manual, often the folders of exported originals beside an app library. See One home for your photos. |
| Backup | A separate copy of your photos that does not change when the library changes. A deletion in the library does not reach it, so it can bring a photo back. See Backups. |
| Bit rot | Damage to a stored file that happens without anyone touching it, so the file no longer opens or opens wrong. A checksum is how it is detected. See Formats and the long term. |
| Camera raw | A file holding what a camera's sensor recorded, before the camera turns it into a JPEG. Each maker has its own format (.nef, .cr3, .arw); it needs special software to view. See Formats and the long term. |
| Card reader | A small device that reads a camera's memory card on a computer, so photos copy without the camera switched on. See Getting photos out. |
| Checksum | A short code calculated from a file's contents. Change one bit and the code changes, so matching checksums mean two files are identical and a changed checksum means a file changed. See Formats and the long term. |
| Conservator | A trained professional who treats damaged photographs and other objects. Stuck, moldy, cracked or early photographs go to one. See Prints, slides and negatives. |
| Copy | A complete, full-size duplicate of the library or part of it, in a different place. A resized copy, a shared album or a placeholder file is not a full copy. See Backups. |
| Copyright | The legal right to copy and publish a creative work such as a photo. It generally belongs to the person who took the photo, not to the owner of a print or the people in it. Questions about it go to a lawyer. See Sharing photos. |
| DNG | Digital Negative, an openly documented raw format from Adobe. Apple ProRAW is built on it. The Library of Congress lists it as acceptable for photographs. See Formats and the long term. |
| Duplicate (exact) | A copy that is byte for byte the same as another file. Exact duplicates share a checksum. See Duplicates. |
| Enclosure | A sleeve, folder, envelope or box that holds a print, slide or negative. Photo-safe enclosures have passed the Photographic Activity Test. See Prints, slides and negatives. |
| Exif | Information a camera or phone writes into a photo file when it is taken: date and time, camera, settings and, if location is on, where. See Dates, captions and people. |
| Export | Saving a copy of photos out of an app into ordinary files. An export can change the format, the size and what information comes with it. See Getting photos out. |
| Export Unmodified Original | Apple's export that gives the file as it was taken or imported, without edits. On a Mac it can write captions and keywords to a separate XMP file. See Getting photos out. |
| Face groups | Google Photos' grouping of photos by the faces in them. The names you give are private to your account. Apple's equivalent is the People and Pets collection. See Dates, captions and people. |
| Favorite | A photo you have marked as one you would most miss. This manual marks favorites before deleting anything. See Choosing what to keep. |
| File extension | The letters after the last period in a file name, such as .heic or .jpg, which usually tell the format. See Formats and the long term. |
| Files On-Demand | OneDrive's setting on Windows that leaves online-only placeholders on the computer instead of the files themselves. See One home for your photos. |
| Fixity | The archivist's word for a file staying unchanged. A fixity check compares checksums over time. See Formats and the long term. |
| Flatbed scanner | A scanner with a flat glass plate the original lies on, face down. The kind archives recommend for prints, rather than a sheet feeder or an office copier. See Prints, slides and negatives. |
| Folder pattern | The rule your folders follow. This manual's is a folder per year with date-first event folders inside it, such as 2026/2026-07-04_lake-trip. See Folders and file names. |
| Free up space | Google Photos' setting that deletes the phone's copies of backed-up photos, leaving the cloud copy at whatever quality it was backed up in. See One home for your photos. |
| Gigabyte (GB) | A measure of storage: 1,000 megabytes as drive makers and cloud services count it. A terabyte (TB) is 1,000 GB. See the storage calculator. |
| HEIC | The lossy photo format iPhones and iPads save when Camera Formats is set to High Efficiency. It is HEIF with HEVC compression. See Formats and the long term. |
| Inactive account | An account nobody has signed in to for a long time. Google, Amazon and Microsoft each have a rule that can delete such an account's photos, with notice. See Taking stock. |
| Inactive Account Manager | Google's setting that shares your data with, or notifies, up to 10 people after a period of inactivity you choose. See Routines and handing on. |
| Incoming folder | A folder kept apart from the library where newly copied photos wait until their count has been checked. See Getting photos out. |
| IPTC | A standard set of fields for what people add to a photo: caption, creator, keywords, people and place shown, copyright. See Dates, captions and people. |
| ISO 8601 | The international standard for writing dates year first: 2026-07-04 or 20260704. Names that start with such a date sort in time order. See Folders and file names. |
| JPEG | The most common photo format. Lossy: saving an edited JPEG compresses it again; opening, copying or captioning it does not. See Formats and the long term. |
| Keep Originals | An iPhone setting (Settings, Apps, Photos) that stops photos being converted to JPEG when they are copied to a computer by cable. See Getting photos out. |
| Leading zero | A zero written in front of a single digit (07, not 7) so that names sort in the right order. See Folders and file names. |
| Legacy contact | Someone you name in an account's settings to reach some of its data after you die. Apple and Facebook offer one; Google's version is the Inactive Account Manager. See Routines and handing on. |
| Live Photo | An iPhone photo with a short video attached. Exported, it becomes two files, a still and a video, that share a name. See Getting photos out. |
| Locked Folder | A Google Photos folder for private photos. Unless its own backup is turned on, its photos live only on that phone. See Taking stock. |
| Lossless | A format that keeps every pixel exactly each time it is saved. TIFF and PNG are lossless. See Formats and the long term. |
| Lossy | A format that throws away some detail to make files smaller, and again each time an edited image is saved. JPEG and HEIC are lossy. See Formats and the long term. |
| Master library | The one place you treat as the real collection. Everything else is a copy of it. See One home for your photos. |
| Metadata | Information about a photo stored with it: when and where it was taken, by what camera, and any caption or keywords added. See Dates, captions and people. |
| Near duplicate | A photo that looks the same as another but is not the same file: resized, re-saved, edited, sent through a chat app, or the next shot in a burst. Near duplicates do not share a checksum. See Duplicates. |
| Offline copy | A backup that is unplugged or disconnected between updates, so a mistake or malicious software on the computer cannot reach it. See Backups. |
| Optimize Storage | Apple's iCloud Photos setting that keeps smaller, space-saving versions on the device and the full-size originals in iCloud. See One home for your photos. |
| Original | The file as the camera or scanner made it, before any edit, resize or conversion. See Formats and the long term. |
| Original quality | Google Photos' backup setting that keeps photos at the resolution they were taken, with no change to quality. See One home for your photos. |
| Over quota | Using more storage than an account allows. Services stop backing up, and some can delete files after months over the limit. See Taking stock. |
| Partner sharing | Google Photos' setting that shares your photos, all of them or chosen faces from a start date, with one other person. See Sharing photos. |
| Path | A file's full address: the drive, every folder above it and its name. Windows programs mostly stop at 260 characters. See Folders and file names. |
| Photographic Activity Test (PAT) | An ISO test of whether a storage material harms photographs. Photo-safe enclosures pass it; "acid-free" alone does not mean that. See Prints, slides and negatives. |
| Placeholder file | An icon that stands for a file kept in the cloud. It takes no space and cannot open offline until it is downloaded. See Backups. |
| PNG | A lossless image format, preferred by the Library of Congress for photographs. See Formats and the long term. |
| ppi | Pixels per inch: how many pixels a scan records for each inch of the original. Twice the ppi gives four times the pixels. See Prints, slides and negatives. |
| Ransomware | Malicious software that locks your files and demands payment. An unplugged copy is out of its reach. See Backups. |
| Readme | A short written description of how the folders and names work, kept with the files. The library plan prints as one. See Folders and file names. |
| Recently Deleted | Apple Photos' holding area for deleted items, kept 30 days before they are gone. Google's equivalent is Trash, also 30 days. See Choosing what to keep. |
| Recovery window | The 30 days a deleted photo stays in Recently Deleted (Apple) or Trash (Google) before it is gone. See Choosing what to keep. |
| Referenced files | Photos a Mac's Photos library points to in another folder instead of holding inside the library. Apple says they are not backed up with the library. See One home for your photos. |
| Relative humidity | How much moisture the air holds compared with the most it could hold at that temperature. Photo storage guidance gives a range of about 30 to 50 percent. See Prints, slides and negatives. |
| Restore test | Opening photos from a backup to prove the copy can be read. This manual's test is yearly, and before you delete anything you believe is backed up. See Backups. |
| Scan | A digital image of a print, slide or negative, made on a scanner. A master scan is kept unedited; copies are made from it. See Prints, slides and negatives. |
| Shared album | An album other people can view and often add to. It is a copy for sharing, not a backup; deleting a shared album can delete its photos for everyone. See Sharing photos. |
| Shared library | One photo library used by several people, such as iCloud Shared Photo Library. Every participant can add, edit and delete. See Sharing photos. |
| Sharing copy | A smaller copy, usually a JPEG, made from a master for sending and posting. Archives call it an access copy. See Prints, slides and negatives. |
| Sidecar file | A separate file beside a photo that holds its information, usually an .xmp file with the same name. See Dates, captions and people. |
| Storage saver | Google Photos' backup setting that resizes photos over 16 MP and may convert them to JPEG. Not a full-size copy. See One home for your photos. |
| Sync | Keeping the same library the same on every device, so a change or deletion on one reaches all. Sync is not a backup. See Backups. |
| Takeout | Google's service for downloading your data, including Google Photos, as .zip or .tgz archives. See Getting photos out. |
| Temporary shared album | An Apple shared album, in version 27 or later, that expires after 30 days and does not use iCloud storage. See Sharing photos. |
| TIFF | A lossless image format used for scan masters by archives and museums. See Prints, slides and negatives. |
| True optical resolution | The resolution a scanner's sensor actually records. A higher "interpolated" figure adds pixels without adding detail. See Prints, slides and negatives. |
| Version | An earlier state of a file that a backup keeps, so a mistake copied into the backup can be undone. See Backups. |
| XMP | A container for photo information that can carry both camera data and people's captions, inside the file or in a sidecar file. See Dates, captions and people. |