A cheer is what you shout at a sporting event when you cheer your team on, showing your enthusiasm and support. Often there's a lot of cheer or good spirits at these games, unless of course your favorite team loses.
The word cheer comes from the Old French chiere, meaning "face." When you cheer someone on, you show approval, whether on your face through a smile, or by shouting encouragement. Cheer is also a quality of being sunny or happy. You can cheer someone up by being silly or entertaining. Or, you could raise a glass to them and toast, "Cheers!"