Pausing from the Operating System:

Linux

If you are on Linux (not tested on OSX)
you can send signals with these commands:

# To pause Blender: killall -s STOP blender # To start it again: killall -s CONT blender 

Some advantages to this are that it will work for any task, not just rendering, and that you do not have to start blender from a terminal.

Windows

You can use process explorer for this. See this question on [StackOverflow]. [StackOverflow]: https://stackoverflow.com/a/15969868/432509

Pausing from Blender:

Pausing F12 renders from Blender is not implemented yet, however it is on the Cycles Todo List.