hook: n. A software or hardware feature included in order to simplify later additions or changes by a user.

:hook: n. A software or hardware feature included in order to
simplify later additions or changes by a user. For example, a
simple program that prints numbers might always print them in base
10, but a more flexible version would let a variable determine what
base to use; setting the variable to 5 would make the program print
numbers in base 5. The variable is a simple hook. An even more
flexible program might examine the variable and treat a value of 16
or less as the base to use, but treat any other number as the
address of a user-supplied routine for printing a number. This is
a {hairy} but powerful hook; one can then write a routine to
print numbers as Roman numerals, say, or as Hebrew characters, and
plug it into the program through the hook. Often the difference
between a good program and a superb one is that the latter has
useful hooks in judiciously chosen places. Both may do the
original job about equally well, but the one with the hooks is much
more flexible for future expansion of capabilities ({EMACS}, for
example, is *all* hooks). The term `user exit' is
synonymous but much more formal and less hackish.
-- The AI Hackers Dictionary