The following video shows how this
is done, and relates it to the slope of the line tangent to y=s(t)
at time t=a. In both cases we get a limit:
limΔt→0s(a+Δt)−s(a)Δt.
This can also be written as:
limx→as(x)−s(a)x−a,
where x=a+Δt.
This quantity (if the limit exists) is called the derivative of s(t) at time t=a.