Video & audio glossary
Plain-English definitions of the video and audio terms behind the tools - codecs, bitrate, resolution and more.
AAC (Advanced Audio Coding)
AAC (Advanced Audio Coding) is a lossy audio codec that shrinks sound by permanently discarding data the ear is least likely to notice. It is the default audio format for MP4 video, Apple devices and most streaming, and at the same bitrate it usually sounds better than MP3.
Aspect Ratio
Aspect ratio is the proportional relationship between a video frame's width and height, written as W:H (for example 16:9). It describes the frame's shape, not its size, and is independent of resolution.
AV1
AV1 is an open, royalty-free video codec from the Alliance for Open Media. It is the most efficient mainstream codec, producing smaller files than H.265 or VP9 at the same quality, at the cost of much slower encoding.
Bitrate
Bitrate is the amount of data used to represent one second of video or audio, measured in kbps or Mbps. It is the main factor controlling both file size and quality - higher bitrate means a larger file and usually better quality.
Codec
A codec (coder-decoder) is software or hardware that compresses video or audio for storage (encoding) and decompresses it for playback (decoding). Common examples are H.264, H.265, VP9, and AV1 for video, and AAC, MP3, and Opus for audio.
Container
A container (or wrapper) is a file format that bundles one or more video, audio, subtitle, and metadata streams into a single file. Examples include MP4, MKV, WebM, MOV, and AVI. A container does not compress the data - that is the codec's job.
CRF (Constant Rate Factor)
CRF (Constant Rate Factor) is the main quality dial in the x264 (H.264) and x265 (H.265) encoders. It targets a constant perceptual quality instead of a fixed bitrate - a lower CRF means higher quality and a bigger file, while a higher CRF means a smaller file with more artifacts.
Frame Rate
Frame rate is how many still images (frames) a video shows each second, measured in fps. Common values are 24 fps for film, 25 or 30 fps for TV and web, and 60 fps for smoother, more fluid motion in gaming and action footage.
H.264 (AVC)
H.264, also called AVC or MPEG-4 Part 10, is the most widely supported video codec in the world. It plays on virtually every device, browser and editor, balancing good quality with near-universal compatibility, and is usually paired with AAC audio inside an MP4 file.
H.265 (HEVC)
H.265, also called HEVC (High Efficiency Video Coding), is a video compression standard and the successor to H.264. It delivers similar visual quality at roughly 40-50% smaller file sizes, making it popular for 4K and high-resolution video.
Keyframe
A keyframe (or I-frame) is a fully self-contained video frame that can be decoded on its own, without referencing any other frame. The frames between keyframes store only what changed, so seeking and clean cuts work best at a keyframe.
Lossless
Lossless compression shrinks a file without discarding any data, so the original can be rebuilt bit-for-bit with zero quality loss. Examples: FLAC, ALAC, and WAV for audio, and PNG for images.
Opus (audio codec)
Opus is a modern, open, royalty-free lossy audio codec that works from low-bitrate speech to high-quality music. It usually beats MP3 and AAC at low bitrates and is the standard audio codec in WebM, used by YouTube and many chat apps.
Resolution
Resolution is the pixel dimensions of a video frame, written as width x height (for example 1920x1080). More pixels mean a sharper, more detailed image, but also a larger file.
Transcoding
Transcoding converts media from one codec or format to another by fully decoding the original and re-encoding it into the target - for example, an H.265 MKV video into an H.264 MP4, or a FLAC audio file into MP3.
VP9
VP9 is an open, royalty-free video codec from Google. It compresses video roughly as efficiently as H.265/HEVC - far smaller than H.264 at the same quality - and is widely used by YouTube and inside WebM files, usually paired with Opus audio.