for an aqueous system in equilibrium,
A <-> C + D
k = [C][D]/[A]
given that any changes in value of [A] is neligible, then [A] can be regarded as a constant, hence [C][D] = k[A] = constant
For aqueous system, A refers to water, C refers to H+ and D refers to OH-. The concentration of water molecules hardly changes in an aqueus system, hence k[A] refers to the ionic product of water.