"(define-key map [t] ...)": What does [t] mean?

[Alan Mackenzie<none@example.invalid>]

Emacs 21.1

I'm trying to understand isearch.el.  Whilst setting up isearch-mode-map
(the keymap), the following function appears to "Make function keys, etc,
exit the search":

(define-key map [t] 'isearch-other-control-char)

What exactly does this "[t]" mean?  I've tried C-h f for define-key, the
key bindings page in the Emacs FAQ, the entire Keymaps section in the
elisp info.  I haven't found the answer.

What exactly does this "[t]" mean?

Thanks in advance.

-- 
Alan Mackenzie (Munich, Germany)
Email: aacm@muuc.dee; to decode, wherever there is a repeated letter
(like "aa"), remove half of them (leaving, say, "a").