jack-o-lantern vs jack-a-lantern

jack-o-lantern and jack-a-lantern both are nouns.

Word	Noun	Adjective	Verb	Adverb
jack-o-lantern	Yes	No	No	No
jack-a-lantern	Yes	No	No	No

As nouns, jack-o-lantern and jack-a-lantern are synonyms defined as:

jack-o-lantern and jack-a-lantern: a large poisonous agaric with orange caps and narrow clustered stalks; the gills are luminescent

Other synonyms of jack-o-lantern include jack-o-lantern fungus, Omphalotus illudens.

jack-o-lantern (noun)	jack-a-lantern (noun)
a large poisonous agaric with orange caps and narrow clustered stalks; the gills are luminescent	a large poisonous agaric with orange caps and narrow clustered stalks; the gills are luminescent