t.
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