megagametophyte vs gametophyte

megagametophyte and gametophyte both are nouns.

Word	Noun	Adjective	Verb	Adverb
megagametophyte	Yes	No	No	No
gametophyte	Yes	No	No	No

As nouns, gametophyte is a hypernym of megagametophyte; that is, gametophyte is a word with a broader meaning than megagametophyte:

megagametophyte: the female gametophyte produced by the megaspore of a plant that produces both microspore and megaspores
gametophyte: the gamete-bearing individual or phase in the life cycle of a plant having alternation of generations

megagametophyte (noun)	gametophyte (noun)
the female gametophyte produced by the megaspore of a plant that produces both microspore and megaspores	the gamete-bearing individual or phase in the life cycle of a plant having alternation of generations