Cue sheet
A small text file describing how a CD is divided into tracks, so a single audio file can reproduce the disc exactly.
The format arrived with the burning tool CDRWIN in the 1990s, where a .cue described the contents of an accompanying .bin image. The lines are human-readable: FILE names the data file, TRACK starts a track and INDEX 01 03:41:22 marks where it begins in minutes, seconds and CD frames, at 75 frames per second. INDEX 00 and PREGAP describe the gaps between tracks, including hidden tracks tucked into the pregap.
Among lossless collectors, a whole disc as one FLAC file plus a cue sheet became the norm, because it preserves the disc's exact structure and plays live albums seamlessly without any special handling.