The ring or stain left on the penis on withdrawal from the anus of your partner. The official "depth mark" is the one closest to the base of your penis and must be at least on 50% around the girth.
Man1: "Dude, I bummed
last night. She was so slack, I've never gone so deep before. I was so impressed I have left the depth mark on, check it out"
*Man1 drops pants, Man2 takes a look.*
Man2: "Jeeze, you must have been able to stick your balls in!"