Homogeneity: If we change the strength of the input by some factor, the output will also change by that factor.

Additivity: If $y_1 = T[x_1(t)]$ and $y_2 = T[x_2(t)]$ then $T[x_1(t) + x_2(t)] = T[x_1(t)] + T[x_2(t)] = y_1 + y_2$ Where $x(t)$ is the input signal, $y(t)$ is the output signal and $T[]$ is the system.

Time invariant systems: A system for which a temporal shift in the input sequence produces a corresponding shift in the output.

Linear time-invariant systems (LTI): Can be represented by the response of the system to a unit pulse. This response is called the impulse response function of the system.