A doorstop is a wedge-shaped object you place under a door to hold it open. Most doorstops are made of rubber, wood, or plastic, and they work by creating friction between the door and the floor. When you slide a doorstop under an open door, it prevents the door from swinging closed, which comes in handy when you're carrying groceries inside, moving furniture between rooms, or just want fresh air flowing through the house.

The word can also describe something that serves a completely different purpose: a very thick, heavy book. When people call a book a doorstop, they usually mean it's so massive and dense that you could literally use it to prop open a door. You might hear someone joke that a 1,000-page history textbook is a real doorstop, especially if they find it boring or difficult to read.