Rebuild Mac OS X whatis database
The "whatis" database is a "set of database files containing short descriptions of system commands" (according to the on-line help) that are commonly used at the Mac OS X command line -- such as in the Terminal application.
The whatis database should be updated periodically to reflect new command-line utilities available on the computer. The command "makewhatis" can be run to do this.
In practice, you should not need to update the whatis database yourself, as it is already refreshed weekly by "Mac OS X maintenance scripts". At the command line in Terminal, type "sudo periodic weekly" to run the particular script that manages the whatis database. There are also several third-party utilities that can help you run the daily, weekly, and monthly maintenance scripts from a graphical application rather than the command line.