Linearity
In my own words, a linear system is a system in which superposition applies. For example, if two inputs x1(t) and x2(t) are applied to the system, the output of the system will the sum of the responses to both inputs, y(t) = y1(t) + y2(t).
For example,
x1(t) + x2(t) $ \rightarrow $ system $ \rightarrow $ y(t) = y1(t) + y2(t)
Linear System
x[n] $ \rightarrow $ system $ \rightarrow $ y[n] = 2x[n]
Proof:
Non-Linear System
x[n] $ \rightarrow $ system $ \rightarrow $ $ y[n] = x[n]^2 $
