A linear expression is an algebraic statement where each term is either a constant or a variable raised to the first power. In other words, none of the exponents can be greater than 1.
For example, x² is a variable raised to the second power, but x is a variable raised to the first power.
5 is an example of a constant.
Note that the coefficients in front of the variables don't matter. Let's take a couple of these polynomials (a polynomial just means an expression with two or more terms) as examples.
2x - y + 3 is a linear expression.
x + y + z⁵ is a non-linear expression. It contains a term raised to the fifth power.
4 - 2 is a linear expression.