Image and PDF glossary

• Alpha channel

  An alpha channel is an extra layer of per-pixel data in an image that records how opaque each pixel is, from fully transparent to fully solid for every pixel it covers.

• Aspect ratio

  Aspect ratio compares an image's width to its height as a simple proportion, describing its shape independently of how many pixels it contains.

• AVIF

  AVIF is a modern image format based on the AV1 video codec that delivers strong compression with lossy, lossless, and HDR options along with transparency and animation.

• Batch processing

  Batch processing is running the same operation across many files at once, such as converting, renaming, or compressing a folder of images with identical settings.

• BMP

  BMP is a bitmap format that typically stores each pixel in its raw form with little or no compression, producing simple, large files used mainly on Windows systems.

• CMYK

  CMYK is a subtractive color model that creates colors by layering cyan, magenta, yellow, and black inks, and it is the standard model for commercial printing.

• Color depth

  Color depth is the number of bits used to store each pixel's color, which determines how many distinct colors an image can represent before banding appears.

• Compression ratio

  Compression ratio is the comparison between an original file size and its compressed size, showing how many times smaller the result is than the input.

• Crop

  Cropping is cutting away part of an image so only a chosen region remains, which changes both the shape of the picture and the number of pixels it holds.

• DPI

  DPI stands for dots per inch and measures how many physical ink or toner dots a printer places within one inch of a page to produce an image.

• EXIF data

  EXIF data is metadata written into a photo file that records details such as camera model, lens, exposure settings, capture time, and sometimes location.

• File extension

  A file extension is the short group of characters after the final dot in a file name that suggests to software what format the file uses.

• File size

  File size is the amount of digital storage a file occupies, measured in bytes and typically expressed in kilobytes or megabytes for images and documents.

• GIF

  GIF is an older image format that stores up to 256 colors per frame, supports simple transparency and looping animation, and compresses best on images with flat colors and few tones.

• Grayscale

  A grayscale image stores only brightness information for each pixel, producing shades from black to white without any color channels.

• HEIC

  HEIC is an image container Apple uses by default since iOS 11 that stores efficient photos, sometimes with several images and depth data in one file.

• Image compression

  Image compression is the process of reducing an image file size by removing or re-encoding data, either without changing visible pixels or by discarding detail the eye rarely notices.

• Image dimensions

  Image dimensions are the width and height of a picture measured in pixels, which together determine how much detail it holds and how large it can display.

• Image format

  An image format is the structure a file uses to store picture data, defining how pixels are compressed, whether transparency is possible, and which programs can open it.

• Image quality

  Image quality describes how faithfully a picture reproduces detail, color, and contrast, and it is shaped by capture, resolution, and compression rather than by file size alone.

• Image resizing

  Image resizing changes the width and height of a picture by adding or removing pixels, which alters file size and display detail while keeping the content of the image.

• Image rotation

  Image rotation turns a picture around its center by a chosen angle, most often 90 or 180 degrees, so it displays upright without changing its content.

• Interpolation

  Interpolation is the method a program uses to calculate new pixel values when an image is enlarged or reduced, based on the pixels that already exist nearby.

• JPEG

  JPEG is a widely supported lossy image format that reduces file size by discarding visual detail that is hardest for the eye to notice, which makes it a common choice for photographs.

• JPEG artifacts

  JPEG artifacts are visible distortions such as blocky patches, ringing around edges, and color blotches that appear when a JPEG image has been compressed too aggressively.

• JPG

  JPG is a three-letter name for the JPEG image format, used because older operating systems allowed only three characters after the dot in a file name.

• Kilobyte (KB)

  A kilobyte is a unit of digital storage equal to 1,000 bytes by international convention or 1,024 bytes in older computing usage, and it is written as KB.

• Lossless compression

  Lossless compression reduces a file size by finding patterns and storing shorter descriptions, so the decoded image is identical to the original with no lost detail.

• Lossy compression

  Lossy compression reduces file size by permanently discarding image data that is least visible to the eye, which produces small files but prevents perfect recovery of the original.

• Megabyte (MB)

  A megabyte is a unit of digital storage equal to about one million bytes, written as MB, and commonly used to express the size of photos, videos, and documents.

• Megapixel

  A megapixel is one million pixels, so a camera or image with a stated megapixel count holds that many samples of color across its grid.

• MIME type

  A MIME type is a standard label such as image/jpeg that tells software what kind of data a file contains, independent of its file extension.

• OCR

  OCR, short for optical character recognition, is the process of reading text from an image or scan and converting it into editable characters that software can search and copy.

• PDF

  PDF is a page-based document format that preserves text, images, and layout exactly as designed so files look the same on any device, printer, or operating system.

• Pixel

  A pixel is the smallest addressable unit of a digital image, a single sample of color and brightness that combines with millions of others to form a picture.

• PNG

  PNG is a lossless image format that stores every pixel exactly as drawn, supports transparent backgrounds, and is commonly used for logos, screenshots, and graphics with sharp edges.

• PPI

  PPI stands for pixels per inch and measures how many pixels a display or image occupies in one inch of physical space, which determines its real-world size and sharpness.

• Raster image

  A raster image stores a picture as a grid of colored pixels, so its detail is fixed at one resolution and enlarging it beyond that grid reveals blocky edges.

• Resolution

  Resolution describes how much detail an image contains, usually measured in pixel dimensions for screens or pixels per inch for print, and it determines how sharp an image can appear.

• RGB

  RGB is an additive color model that builds colors by combining red, green, and blue light, which is how monitors, phones, and cameras represent images.

• Static WebP

  Static WebP is a single-frame image in the WebP format with no animation, so it behaves like a JPEG or PNG and can be converted and compressed like other still images.

• SVG

  SVG is an XML-based vector format that describes images as shapes, paths, and text, so graphics stay sharp at any size and files remain small for simple artwork.

• Thumbnail

  A thumbnail is a small copy of a larger image, usually a few hundred pixels wide, used as a quick preview in galleries, search results, and file lists for images.

• TIFF

  TIFF is a flexible container format for high-quality images that supports lossless storage, high color depth, layers, and multiple pages, and is widely used in scanning and print work.

• Transparency

  Transparency is an image property that lets some pixels show the background or layers beneath them instead of painting a solid color, and it is stored through an alpha channel.

• Upload limit

  An upload limit is the maximum file size, dimension count, or format a website or form accepts before it refuses a file as too large or unsupported.

• Vector image

  A vector image describes graphics as shapes, paths, and text defined by coordinates, so the artwork can be scaled to any size without losing sharpness.

• Watermark

  A watermark is text, a logo, or a pattern overlaid on an image or document to show ownership or source, either visibly across the picture or hidden within it.

• WebP

  WebP is a modern image format, released by Google in 2010, that offers both lossy and lossless compression plus transparency, usually at smaller file sizes than older formats.

• ZIP file

  A ZIP file is an archive that packages multiple files into one, compressing each one with a lossless method so the bundle takes less space and travels as a single item.

Put the definitions to work

Compression, resolution, and format choices are easier to compare side by side, and every task has a free tool on this site.

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