The phenomenon that occurs when photos are taken at the optimal position on the z plane in relation to cleavage to give the illusion of 1.5x - 3.0x the actual cleavage. This is mostly done by illuminating the front of the cleavage and casting shadows on the back and/or top of the cleavage, thereby creating the affect of depth, volume, and possibly atmospheric perspective.
*Note: photos w/ flash cleavage are 12x more likely to be posted on the internet by the cleavage owner; who will then pretend to not know what you are talking about approximately 100% of the time.
Matt: dude here's a pic of that girl Madison I met at the club...
Tyler: bro am I detecting boob job???
Matt: ...nahnahnah, there's just maximum flash cleavage there.
Jeremi: nice bro.