expect (re: rexec)

In http://forums.unix.com/showthread.php?threadid=391
there is one statement called expect.
but when I issue command whereis expect, respond from o/s only EXPECT: (only one world).
I try to find it at /usr/bin, no expect statement there ?
is it default unix o/s command ?
I am using AIX on RS6000
thanks for your kind answer

expect is not just a command. Expect is a tool for automating interactive applications such as telnet, ftp, passwd, fsck, rlogin, tip, etc. Expect really makes this stuff trivial. Expect is also useful for testing these same applications. And by adding Tk, you can also wrap interactive applications in X11 GUIs.

and it is not default Unix tool(newest Linux distributions Contains Expect)

you will get your expect copy form: