Geometrical Meaning
Let P(x,y) and Q(x + Dx,y + Dy) be two neighbouring points on the curve y = f (x).Let TPT' be the tangent at P, PQ is called the secant of the curve and the slope of secant 
.
Physical Significance
Let Q (t) be a quantity that changes with time 't'.Let Dt be the increment given to 't' and DQ be the corresponding increment in Q, then 
is called the average rate of change of Q with respect to 't'. (Also known as Newton - quotient of f at t)
Note:
If 's' is the displacement of a particle at time 't' and let s = Q(t),
