The region between a woman's thighs which permits air to pass through freely-or not to pass through at all...whatever the case may be. Air flow is contingent on the size of any given woman. i.e. fat girls have no flow, whereas thin girls have great flow.
Can be modified to fit every situation.
1.) She's got great air flow. I'd love to tap that.
2.) Yes she's got a cute face, butter flow. (this would insinuate that the girl is a bit larger, and has no flow. No flow increases the likelihood of a girl having V.O.)
3.) No flow ho's have the worst V.O.