The boolean representing true, and the canonical generalized boolean representing true. Although any object other than nil is considered true, t is generally used when there is no special reason to prefer one such object over another.
The symbol t is also sometimes used for other purposes as well. For example, as the name of a class, as a designator (e.g., a stream designator) or as a special symbol for some syntactic reason (e.g., in case and typecase to label the otherwise-clause).
t => T (eq t 't) => true (find-class 't) => #<CLASS T 610703333> (case 'a (a 1) (t 2)) => 1 (case 'b (a 1) (t 2)) => 2 (prin1 'hello t) >> HELLO => HELLO