What's the difference between a progestin and progesterone?

Biology. Progesterone is either produced in a woman's ovary or synthesized in a lab and is considered "bio-identical". A progestin, as dr eun mentions, is biologically similar but not identical. But don't be fooled by the labels, "natural" and "synthetic". No plant ever produces progesterone! that is identical to a human woman's!
Similar. Progesterone is a specific hormone produced in the ovary. Progestin compounds are all molecules that have progestational effect which includes progesterone. Different progestin compounds have different biological activity.