Hydrogen Atom and Relatavistic Corrections - What does this Alpha mean

Homework Statement

Okay, I need to work out the Relativistic corrections for an atom, using the First KE Term and the Darwin Term:

Calculate the energy shift of the protonium groundstate due to the relativistic correction to the kinetic energy and the Darwin term and compare it to Hydrogen

Homework Equations

KE Term:

$$\Delta E_1 = \frac{1}{2}mc^2 \frac{(z\alpha)^4}{n^4}\left( \frac{n}{l + \frac{1}{2}} - \frac{3}{4}\right)$$

Darwin Term:

$$\Delta E_3 = \frac{1}{2}mc^2 \frac{(z\alpha)^4}{n^3}$$

The Attempt at a Solution

I know I just need to insert the values into these equations to get out the answer. However I am niot quite sure what the alpha represents?

TFM

$$\alpha = \frac{e^2}{4\pi\epsilon_0 \hbar c} \approx \frac{1}{137}$$