A linear system is a system for which if you can add two functions and multiply them by scalars then pass them through the system, it is equivalent to passing the two signals through the system and then adding them and multiplying them by scalars.
Example of Linear System
System --> z(t) = x(2t)
X1(t) --> Y1(t) = 4X1(t)
X2(t) --> Y2(t) = 3X2(t)
W(t) = Y1(t) + Y2(t) = 4X1(t) + 3X2(t) --> System --> Z(t) = 4X1(2t) + 3X2(2t) (1)
X1(t) --> System --> Y1(2t) --> multiply by 4 --> 4Y1(2t)
X2(t) --> System --> Y2(2t) --> multiply by 3 --> 3Y2(2t)
Z(t) = Y1 + Y2 = 4Y1(2t) + 3Y2(2t) (2)
Equations (1) and (2) are the same so therefore it is a linear system.
