Decaf is everyday, slightly informal English for coffee — sometimes tea — with the caffeine taken out, or mostly taken out. People say 'I'll have a decaf' meaning a cup of that coffee. Cafés write it on menus. You can use it as a noun (a decaf) or in front of another noun (decaf coffee). Though actually, decaf is rarely zero caffeine — a little is usually still there — but it is far less than a normal cup, which is why people drink it in the evening. The full word is decaffeinated; decaf is the short, spoken form.