Where does the word nativity come from?

Nativity derives from the Latin 'natal' meaning birth. 

Nativity scene in Barcelona

Natal is also the source of several other English words, including native and nature.

While pre-natal is the standard medical term and in common use, nativity is almost exclusively used in the religious/iconic sense. This is defined in the Oxford Dictionary as "the birth of Jesus Christ/a picture, carving, or model representing Jesus Christ’s birth".

The Nativity story is based on two biblical accounts. One is in the gospel of Luke and the other in that of Matthew. Both describe Jesus being born in Bethlehem, in Judea, to a virgin mother during the reign of King Herod. They also agree that his mother was named Mary and that her husband Joseph was descended from King David.

On other aspects there are significant differences between the two versions - see here