Correct definition of algebraic expression

An algebraic expression is a mathematical expression that contains constants and/or variables that can be expressed using letters and/or numbers.

For example, the following expressions are algebraic:

2x + 4 = 5 (Where 'x' is a variable) 2x + a = 3 (Where 'x' is a variable and 'a' a constant) a + bx + cy = 1 (Where 'x' and 'y' are variables and 'a', 'b' and 'c' are constants).

The following are NOT algebraic expressions

π e