apropos

This command will give you a short definition of a topic.

Example:

apropos rxvt
rxvt (ouR XVT) [rxvt] (1) - a VT102 emulator for the X window system

If you use apropos with a topic that has more than one option it will list the man pages that are available.  So you can choose one specific topic from the list.

 apropos bash
bash (1)             - GNU Bourne-Again SHell
bash-builtins (7)    - bash built-in commands, see bash(1)
bashbug (1)          - report a bug in bash
builtins (7)         - bash built-in commands, see bash(1)
dh_bash-completion (1) - install bash completions for package
ispellconfig (8)     - Bash script to select a new ispell default dictionary.
rbash (1)            - restricted bash, see bash(1)
update-ispell-dictionary (8) - Bash script to select a new ispell default dic..