The 2D image that represents a character, an enemy or a coin in a game and moves over the background.
A sprite is any image that moves in a 2D game: the hero, an enemy, a coin, a bullet. The movement itself is made by showing several drawings quickly one after another, say one for standing, four for running and two for jumping. These drawings are usually packed into a single image called a sprite sheet.
How a sprite looks and moves is the first thing a player notices, and it gives the game its personality. But beginners often get stuck drawing and never reach the playing part. In 2D Game Design you start with a working game that has ready characters and colors you can change, so you test the idea before worrying about looks. Practical tip: make the game fun with simple shapes first. If it is not fun with colored squares, beautiful art will not save it; if it is fun, improving the art afterwards is easy.