What is the proper term for a Marine who is no longer serving?

Marine.

SFC Richard Desrosiers - Richard, it's just a bit hard for someone who was trained and expected to value being a 'soldier' to hear this stuff ad infinitum. I was drafted, expected VietNam. Denigrating a soldier was something that we grunts supposed selfish, unpatriotic folks did. I hope you all didn't mean to do that? Interservice rivalry probably killed more Americans than the bad guys did, truth be known -- and I mean with the Pentagon types, too! Sad, man.

You can call me Sir, and then make yourself useful and fetch me a beer.

MGySgt Mendez: I was going to join the Marines until they found out my parents were married when I was conceived; then after 4 years in the Navy - they let me go because I grew over 6’ tall. Someone said now go join a real service - The Coast Guard!

All ex-military, Marines, Air Force, Army, Navy are referred to as veteran's. When asked about my status I always say VETERAN, U.S. Army. I am a soldier for life. Once discharge and no longer active or in-active duty you are a veteran. I would never call you a Marine, Air Man, Soldier or Seaman first.

I put on my uniform
I wore it to foreign lands
The soldier I was
Is still in the man
I have been a husband, father, and friend
But I've been a soldier all along
Even after so many a year
My final salute
I render today
I'm still a soldier
I'm just on my way. - Timothy Emmons Drafted 1967

SSgt Danny Radcliffe - The term for a Marine that voluntarily decided he or she no longer wanted to serve on active duty(and there's no shame in that decision), is Former Marine. If ones ego is so large until they cant face that fact, I suggest you either re-join or never get out in the first place

SSG Richard Bladl - you forgot to include the U. S. Coast Guard. We fall into the same category of former members of the Armed Services as well.

CPT (Join to see) - I was NOT SHOUTING...
That is my Marine Voice. Sorry I made you jump, it happens!
People usually say Marine...
Some just say, Vet.
Ass female I usually not address as Vet but think I am tagging along with my Marine Husband (also Veteran)

Cpl Rosemarie SantaAnna -

Same. Drives me nuts when the marines in my guard unit claim to be " combat marines" from our deployment

CPT (Join to see) - What did you say