Cross-notch 

Two people share a cross-notch when they have both had the same sexual partner. A cross-notch can originate in a threesome or when two sexual encounters occur at different times but with a common party in each. A cross-notch can be shared between friends or enemies, but is most often cataloged when between friends. Some friends might even get three or four different cross-notches during the course of a lifetime.

The term originates from the notch put into the handle of a gun following a confirmed kill, serving as a form of score keeping.

"Jerred and James finally logged the first cross-notch

of their friendship when Amanda, the giving downstairs neighbor, quit laying with James and spent a night in Jerred's room."

Cross-notch by Wesley Crusher January 28, 2013