File Glossary

Definitions for file formats, codecs, compression standards, and technical terms — from A to Z.

A

Alpha Channel: An additional channel in an image that stores per-pixel transparency information, enabling backgrounds to show through.
Aspect Ratio: The proportional relationship between an image's width and height, expressed as width:height (e.g. 16:9 or 4:3).

Bit Depth: The number of bits used to represent each colour channel in an image or audio sample. More bits = more possible values = finer gradations. Common values: 8-bit (256 values), 16-bit (65,536 values), 24-bit (audio).
Bitrate: The amount of data used per second in a media file — higher bitrate means better quality but larger file size.

Checksum / File Hash: A calculated value derived from a file's contents — used to verify the file has not been corrupted or tampered with during transfer.
Codec: Software that encodes and decodes digital media — compressing video or audio for storage and decompressing it for playback.
Compression Ratio: The ratio of a file's original size to its compressed size — a measure of how effectively a compression algorithm reduces file size.
Container Format: A file format that bundles one or more video, audio, subtitle, or metadata streams into a single file. Examples: MP4, MKV, MOV, AVI, OGG. The container is separate from the codec used to encode the streams inside it.
Cross-Reference Table (xref): The internal index inside a PDF file that maps object numbers to their byte positions — the most common point of PDF corruption.

DPI (Dots Per Inch): A measure of image resolution — the number of pixels or printed dots per inch. Higher DPI means more detail and larger file sizes.

EXIF Data: Metadata automatically embedded in JPEG photographs containing camera settings, GPS location, date, and other capture information.

File Corruption: Damage to a file's internal data structure that makes it unreadable or causes it to produce errors when opened.
File Extension: The suffix at the end of a filename (e.g. .pdf, .jpg, .zip) that indicates the file's format and which application should open it.
File Fragmentation: When a file's data is stored in non-contiguous chunks across a storage device, potentially slowing access and increasing corruption risk.
File Header / Magic Bytes: The first few bytes of a file that identify its format — used by applications to verify file type independent of the file extension.
Frame Rate: The number of individual images (frames) displayed per second in a video, measured in fps (frames per second). Common values: 24fps (cinematic), 30fps (TV), 60fps (sports/gaming), 120fps (slow-motion).

Image Resolution: The number of pixels in an image, typically expressed as width × height (e.g., 1920×1080) or total megapixels. Higher resolution means more detail and a larger file size.

Keyframe: A complete video frame stored without reference to other frames. Video codecs only save full data every few seconds (keyframes); between them, only the changes are stored (delta frames). Seeking in a video jumps to the nearest keyframe.

Lossless Compression: A compression method that reduces file size without discarding any data — the original file is reconstructed exactly from the compressed version.
Lossless vs Lossy Compression: Lossless compression reduces file size without discarding any data — the original is perfectly reconstructed on decompression. Lossy compression permanently discards data to achieve smaller sizes, trading quality for smaller files.
Lossy Compression: A compression method that permanently discards some data to achieve smaller file sizes — the original cannot be exactly reconstructed.

Metadata: Data stored inside a file that describes the file itself — including author, creation date, camera settings, GPS location, and more.
MIME Type: A standardised label that identifies the type and format of a file, used by web servers and browsers to handle files correctly.

OCR (Optical Character Recognition): Technology that extracts machine-readable text from scanned images or image-based PDFs, making content searchable and selectable.

Progressive vs Baseline JPEG: A progressive JPEG loads in low-resolution layers that improve with each pass — the full image appears blurry then sharpens. A baseline JPEG loads top-to-bottom in a single pass.

Sample Rate: The number of audio samples recorded per second, measured in Hz or kHz. Higher sample rates capture higher frequencies. CD audio uses 44,100 Hz (44.1 kHz); professional recording uses 96,000 Hz (96 kHz).

Transparency / Alpha Transparency: The ability of an image to have pixels that are partially or fully invisible, allowing backgrounds to show through. Supported by PNG, WebP, GIF (1-bit only), and AVIF — but NOT by JPEG.

Vector Graphics: Images defined by mathematical equations (lines, curves, shapes) rather than pixels — they scale to any size without losing quality.