Final answer:
Hybrid sterility is a postzygotic barrier where the hybrid offspring are unable to produce fertile or viable offspring.
Step-by-step explanation:
When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. In the case where the fertile hybrid offspring fail to produce fertile or viable offspring, this is known as hybrid sterility. Hybrid sterility refers to the situation where the hybrid individuals are unable to reproduce offspring of their own. This is an example of a postzygotic barrier, as it occurs after zygote formation.