Oh, come on, people! It means "Before the Common Era," and it's at least a step in the direction of recognizing that not everyone on Earth (or even in this country) (a) believes in Christ and (b) wants him shoved down their throats whenever they're talking about the days before Christianity became a b/l/i/g/h/t/ o/n/ t/h/e/ w/o/r/l/d/ presence in history.

Back from whence you came, heathen.