A Boombardment is when you drop down on the remaining survivors, as a boomer, in Left 4 Dead or Left 4 Dead 2. This will turn out as a massive explosion of boomer bile, which attracts the horde.
A Boombardment is a good tactical move if your move is low on health and it about to die anyway.
Guy #1: OMG, did you just see that?! I used the Boombardment trick and I covered all four survivors in boomer bile!!!
Guy #2: Yeah, that was amazing, it gave the other infected a chance to actually attack those guys and beat the living crap out of them!!