Terminal kill command

I am having a problem with an app. I am trying to kill it in terminal.

I was given a very simple explanation and example by a very experienced friend of mine. But I forgot it already.

he said "go into terminal. type in 'top' fin the process that is using all your cpu then kill it by typing........ " that is where it go fuzzy. something about there were four letters, then 'kill', then the process number (254 in this case)

Can any of you Mac experts tell me what that simple code was????????