Paint is a basketball term that refers to the area on the court known as the Key. It is found directly under the basket. It ends at the top of the key and starts at the endline.
It is called the ‘Paint’ because it is usually painted a different color from the rest of the court. It is the most active area on the court and most of the action in the game is played in the paint.
In the NBA, the paint is usually 16 feet by 19 feet.