The actual indentation left in a piece of furniture when your butt spends way too much time on it. On some types of furniture (especially the cheap kind that you'll find in my house) these dents are permanent and no amount of cushion fluffing will put them right again. This has caused some friction with past roommates because my butt is a couple sizes smaller than the average joe and makes the places I frequently roost uncomfortable sitting for folks with standard sized butts.