When did Euclidean Fifth Postulate comes to be called the Parallel Postulate? It is taken for granted. Did Euclid refer to the fifth postulate as the Parallel Postulate?
The fifth postulate does not specifically talk about parallel lines, it is only a postulate related to parallelism. Euclid gave the definition of parallel lines in Book I, Definition 23 just before the five postulates. However, we see that there is the following equivalence:
About the history of the fifth postulate see the article
History of the Parallel Postulate by Florence P. Lewis.