Prolonged, organised movement carried out in a car, usually restricted to the arms and head. Dancing to the YMCA or other such party dances are accepted routines for car aerobics.
Car aerobics are usually carried out in a moving car and is an accepted form of exercise.
If multiple people are doing the same car aerobics routine at the same time, this is called synchronised car aerobics, which is both an art and a skill.
Guy #1: Hey, wanna come back to mine and play some Fifa?
Guy #2: Nah, I'm not really into that. I'd rather do some car aerobics on my way home.
Guy #1: I wish I was as motivated to exercise as you.