Linear System
A system is called "Linear" if for any constants a,b and for any inputs x1(t),x2(t),(x1[n],x2[n]) yielding output y1(t),y2(t),respectively, the respond to a*x1(t)+b*x2(t) is a*y1(t)+b*y2(t)
Example
Let:
x1(t)=t, x2(t)=t+1;
y1(t)=2*1(t)= 2*t, y2(t)=x(t)+1= t+2;
a=2, b=3;
so, a*x1(t)+b*x2(t)=2*t+3*(t+1)=5*t+6
