In algebra, a constant is a term that contains only numbers and no variables (letters). Unlike a variable, the value of a constant never changes.
An algebraic expression consists of variables and constants written in a phrase. For example, x+3 is an algebraic expression. The x is a variable because it can have more than one value, but 3 is a constant because its value is always the same.
Examining an expression makes it easier to understand the difference between variables and constants. In the expression 2x+2, it is possible to replace x with any real number. The value of 2x depends on what value is assigned to x. The 2 in the second part of the expression never changes, which is what makes it a constant.