What distinguishes a linear function from other functions?

A linear function is one in which the power of the function is only one. So, the graph of it would be a straight line. For example, x2 + x = y is not linear, because the highest power is 2. A main difference is, non linear functions have curves, where as a linear function is a straight line, with the exception of when the function has a power of 0, and it is technically a straight line.