The number of generations produced by an organism per year.
The term is most often used in relation to insects.
Etymology: Italian volt, turn or time + Latin suffix -ine (of or pertaining to)
The wikipedia article lists words for specific numbers of generations per year (univoltine, semivoltine, etc.).
And here's an article on Monarch butterfly voltinism. (It takes multiple generations for Monarch butterflies to complete their annual migration.)