At the gym, "lat" is short for this broad, flat muscle on each side of your back
Ideally, to do lat 
pulldowns you'll want to use a lat pulldown machine at your gym,... [DOC] [TLE] Latissimus dorsi (Lats) Exercises & Workouts - FreeTrainers.com [PAR] The muscle is a large group found on your back and referred to by the Greeks as 
broad muscle of the back.