A cheer is an expression of happiness or support for someone.
When you hear people cheering at a sports game or concert, they're expressing their enthusiasm and excitement. Cheering can also mean to shout out words of encouragement, like "Go team!" or "You got this!". Even something as simple as saying "I'm so happy for you" when a friend achieves something is a form of cheer. Some people might use the word to describe a loud, energetic noise, like a crowd cheering on their favorite musician at a festival.
How common is "cheer"?
Word cheer is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
(uncountable) A cheerful attitude; happiness; a good, happy, or positive mood.
(now uncountable) That which promotes good spirits or cheerfulness, especially food and entertainment prepared for a festive occasion.
Example: a table loaded with good cheer
(countable) A cry expressing joy, approval or support, such as "hurrah".
Example: Three cheers for the birthday boy!
(countable) A chant made in support of a team at a sports event.
(obsolete) One's facial expression or countenance.