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.
Aegisub's native subtitle format is Advanced SubStation Alpha text, which supports subtitle positioning and styling.
In fansubbing terms, Aegisub is used for translating, timing, editing, typesetting, quality checking, karaoke timing and karaoke effecting.