Not limited to running exclusively, but the foul odor/moisture that is present in the pubis region of a female post-exercise.
The severity of Track Twat is directly proportional to the amount of pubic hair contained within the area in question.
"I found out the girl that I run with is a swinger. I'd totally bang her mid-run if it weren't for TRACK TWAT."
"I totally wanted to wear these panties out tonight, but I can't because of TRACK TWAT!"