What is Stacking, root-mean-square and interval velocities?

The interval velocity $v_i$ of a layer is its own speed; with $\Delta t_i = 2h_i/v_i$ the two-way time across it, the root-mean-square velocity down to the base of layer $n$ is

The stacking velocity of a reflection is the speed $V$ of the hyperbola $t^2 = t_0^2 + x^2/V^2$ that best fits its times in a CMP gather.