What is dissociation constant?

In Chemistry, the dissociation constant describes a dissociation reaction in which a compound is broken up. It measures how likely the reaction will occur. For example for reaction AB - > A + B, the dissociation constant is equal to concentration of A x concentration of B / concentration of AB at equilibrium.

for reversible reaction AxBy xA + yB

the dissociation constant = [A}^x [B]^y / [AxBy]

where [A], [B] and [AB] are concentrations of each in equilibrium.