I created an example you can find here
Basically, it's when a character is sitting on a flat surface where one leg is out straight (not illustrated in link) and the other one is propped-up so the character can rest their arm on it, pose for "The Thinker", etc.
I'd like to look up some pictures here for reference for one of my next pieces (and rest assured, I won't upload it unless Vincent van Gogh himself rises from his grave and gives it an 18th-century equivalent of a thumbs-up. You'll be able to find it on my website.)
Updated by Circeus