Cnidarians have a complex life cycle which usually goes as follows:
Egg hatches larva.
Larva settles on sea bottom and transforms into a sessile polyps.
Polyps undergoes strobilation and asexually buds off juvenile, free-swimming medusae.
Medusae eventually mature and reproduce sexually to beget new eggs.
Given the morphological and physiological disparity between the three forms (larva, polyps, medusa), and how each form comprises a distinct stage in the cnidarian's development (i.e. ontogeny), would it be accurate to say that the cnidarian life cycle is an example of "ontogenic polymorphism"?
Sure it would. I would usually use holometabolous insects as an example of this (egg, larva, pupa, adult), but Cnidarians certainly fulfill the basic requirements encapsulated by the phrase "ontogenic polymorphism." It appears that the pressures of finding/establishing food sources and escaping predation in competitive environments has driven both Cnidaria and Insecta toward this evolutionary strategy, though that is a field of active study still (at least in insects).