DAT is a system, popular in the late 1980s and 1990s, for recording sound (and sometimes other data) in digital form onto a small cassette. Studios liked it because the copy stayed clean — numbers, not the hiss of old analog tape. You say the letters one by one: D-A-T. The cassette is smaller than a household analog cassette. Hard-disk recording, CDs, and computer files have mostly replaced it, so you meet the word now in music-history talk, old studio gear, and collector circles more than in new shops.