Vanilla tahitensis (aka Vanilla x tahitensis) is a natural hybrid between two vanilla species. A long time ago, an insect pollinator (maybe a bee) carried pollen from a V. odorata flower and transferred it to a V. planifolia flower. The bean ripened and seeds eventually grew from that bean resulting in what we call Vanilla x tahitensis (the "x" means it's a hybrid).
Today, V. x tahitensis is grown commercially in places like Tahiti, Papua New Guinea, and Ecuador. Along with V. planifolia, it is one of two types of vanilla permissible for use in vanilla extract.