What is the word that marines say to each other?

We say a lot of things to each other so you'd have to be more specific. "Err" can be a greeting or a kind of acknowledgement. "Hoorah" is also used. "Errah" is another form of 'Hoorah'. We also say "Kill" which can be an acknowledgement as well. "Semper Fidelis" or just "Semper Fi" is also a term that is frequently used. It is our official motto which in Latin says "Always Faithful".