Refers to the fold of delicate skin visible between the character's torso and arm that can still be seen when the inner portion of the armpit isn't visible, e.g. because the arm is lowered or the armpit is turned away from the viewer's perspective. See post #2958458, post #3398674, and post #3695398 for examples of what should be tagged as an armpit crease.
This tag was borne of efforts to disambiguate armpit peek (forum #140761) and was never intended to be used for completely visible armpits. For images where the entire armpit can be seen, just use the armpits tag.
Erroneously called an 'armpit vagina'.