How about "Intergluteal Cleft". After searching in an online medical dictionary (Dorland's Illustrated Medical Dictionary), this seemed to make the most sense.
crena (cre-na) (kre¢n[schwa]) pl. cre¢nae [L. "notch," from crenare to split] a notch or cleft
crena ana¢lis, [TA] intergluteal cleft: the cleft between the buttocks on which the anus opens. Called also anal, clunial, gluteal, or natal cleft, c. ani or c. interglutealis [TA alternatives], c. clunium, gluteal furrow, rima ani, and rima clunium.
gluteal sulcus, sulcus glutea¢lis, [TA] a curved transverse groove or fold on the posterior aspect of the upper thigh, separating the upper part of the thigh from the nates (buttocks); called also gluteal fold or furrow.
Butt crack is so much more descriptive though, wouldn't you agree?