What is Degree of dissociation?

The degree of dissociation of a weak acid HA at analytical concentration $C$ is $\alpha = [\mathrm{A}]/C$, the fraction of the acid dissociated by water; it satisfies Ostwald’s dilution law $\alpha^2C/(1 - \alpha) = K_a$, and tends to 1 as $C \to 0$.