x2 + 6x + 9
Above is an example of an algebraic expression. Algebraic expressions are expressions which contain terms, variables and coefficients, and constants.
Terms are the elements in an algebraic expression, separated by the arithmetic signs.
A term may consist of variables and coefficients, or a constant.
The letters in a term (by extension, algebraic expression) are called variables.
Coefficients are the numbers before variables.
Constants are the terms that contain numbers only.