One plant or two
A plant with unisexual flowers raises a second question: are both kinds on the same plant, or on different ones?
Monoecious plants
If both kinds of flower — staminate and pistillate — arise on the same plant, those plants are called monoecious plants.
Examples — pumpkin, maize, coconut, bitter gourd.
Dioecious plants
If staminate flowers and pistillate flowers arise on two separate plants, those plants are called dioecious plants.
Examples — papaw, Vallisneria.
Why a papaw grower counts trees
A dioecious plant cannot pollinate itself at all — the pollen and the ovules are on different individuals. A papaw grower with only male trees will never get fruit, however healthy the plantation looks, and a field of only female trees will not fruit either.
Two words, easily confused
Unisexual and bisexual describe a flower. Monoecious and dioecious describe a plant. Only a plant with unisexual flowers can be called monoecious or dioecious; the question does not arise for a hibiscus.