Linearity
A system is linear if the responses to inputs multiplied by constants are the original responses multiplied by the same constants.
In symbols:
If
$ y1(t)+y2(t)\, $ are the responses to inputs $ x1(t)+x2(t)\, $
Then
$ a*x1(t)+b*x2(t)\, $ should yield $ a*y1(t)+b*y2(t)\, $
Example
A very simple linear system is $ y(t) = t*x(t)\, $
If $ x(t) = t^2\, $, then $ y(t) = t^3\, $
Multiplying $ x(t)\, $ by a constant $ a\, $ yields $ y(t) = t*x(t) = t*a*t^2 = a*t^3\, $
This fulfills the requirements that $ a*x(t)\, $ yields $ a*y(t)\, $
