Image & PDF Glossary
A plain-language reference for the technical terms you'll encounter when working with images and documents.
Working with images and PDFs means running into a lot of jargon. This glossary explains the most common terms in plain language, so you can make better decisions about formats, quality, and compression.
Lossy compression
A method that reduces file size by permanently discarding some image data the eye is unlikely to notice. Used by JPG and lossy WEBP. Produces small files but quality degrades if repeated.
Lossless compression
A method that reduces file size while preserving every pixel exactly. Used by PNG and lossless WEBP. Savings are smaller but there is no quality loss.
DPI / PPI
Dots (or pixels) per inch — a measure of pixel density. Relevant for printing; on screen the total pixel dimensions matter more than DPI.
Alpha channel
An extra layer of data that defines transparency for each pixel, letting parts of an image be see-through. Supported by PNG and WEBP, not by JPG.
Artefact
A visible flaw introduced by compression, such as blocky patches or halos around sharp edges, more noticeable at low quality settings.
EXIF
Metadata embedded in photos recording camera settings, date, and often GPS location. Can be stripped by re-encoding the image.
Resolution
The pixel dimensions of an image, expressed as width × height. Higher resolution means more detail and larger files.
Aspect ratio
The proportional relationship between an image's width and height, such as 16:9 or 4:3. Preserving it prevents distortion when resizing.
Canvas
A browser drawing surface used by JavaScript to render and export images. The conversions on this site use it to process files locally.
Bitmap / raster
An image made of a grid of coloured pixels (JPG, PNG, WEBP). Contrasts with vector images, which are defined by mathematical shapes.