People who boost for extra points, emblems and other in-game valuables. People who do this act like a baby . For them there is no other way, than a childish and nooby way, to earn in-game experience points.
Boosting is done by at least two people, so most of times people use the plural, Baby Boosters. Sometimes it's done by more than 2 people, that's called a baby booster gang bang.
Quote from TheGervan: There's a baby booster gang bang goin on.