ab crack

plural ab cracks

1. colloquialbodybuildingThe vertical ridge visible between a person's abdominal muscles, when highly toned.

   Any self-respecting It girl of the moment knows that the photo the paparazzi want is the one in gym leggings and a crop top, exposing this year’s must-have ab-crack.

   2016 November 1, Jess Cartner-Morley, The Guardian

   1 more example

   And now it's the turn of the horribly painful sounding 'ab crack' which has got people talking. That line going down the middle of the abdominals of most self-respecting bikini models? That’s what it is.

   2016 July 20, Sian Ranscombe, The Telegraph