1. Name

apropos - search the manual page names and descriptions

2. Synopsis

apropos  [-dhV] [-e|-w|-r] [-s section]
 [-m system[,...]] [-M path] [-C file] keyword ... 

3. Frequently used options

-s section, --section section   Search only 
the given manual section. 
-e, --exact  Each  keyword will be exactly matched
against the page names and the descriptions. 

4. Examples

This command searches keyword in command description. What commands are related to partitions:

$ apropos partition 

or what was the command that I used yesterday to scan all ports on my server:

$ apropos scanner 

search for commands only in the manual section 1

$ apropos -s 1 partition 

match only exact words:

$ apropos -e scan 

number of outputs with and without -e option: