Aegisub is a free open-source cross-platform subtitle editing program. It is extensively used in fansubbing, the practice of creating or translating unofficial, noncommercial subtitles for visual media by fans. It is the successor of the original SubStation Alpha and Sabbu.
The software is considered the standard in a well-known fansubbing group, and has been recommended in the online "Guide to Fansubbing".
It has been designed for timing and styling of subtitles, as well as the creation of karaoke. Aegisub's native subtitle format is Advanced SubStation Alpha text, which supports subtitle positioning and styling. The program also supports other common formats such as SubRip. Features include support for timing to both audio and video, and can use many video processing bindings to process those, such as FFmpeg and Avisynth. It can also be extended with the Lua, Perl and Ruby scripting languages
In fansubbing terms, Aegisub is used for translating, timing, editing, typesetting, quality checking, karaoke timing and karaoke effecting. Although, many groups use different tools for some of those steps, such as Adobe After Effects for typesetting, or a simple text editor for translation.