linearity

A rule is linear when scaling or adding its inputs scales or adds its outputs the same way -- no exponents, products of inputs, or added constants.

An operation is linear when it satisfies superposition -- scaling an input by $c$ scales the output by $c$ , and adding two inputs adds their outputs, with no leftover constant or cross term. This same property, applied specifically to systems that also don't shift over time, is what makes an LTI system tractable.