Is the term soldier meant only for army or is it meant for anybody in the military?

Ok, I was watching Letters from iwo jima earlier and a us marine got cuaght as a POW but the japanese called him a soldier. I was like thats dumb that that called him a soldier when he was a marine but my step dad said " A soldier is a term for any branch in the military" even though he has know military experience at all. I said no (corect me if im wrong) Navy are sailors, marines are marines Army is soldiers...... Don't know the USAF though..... Waddya think?