headword vs head word

headword and head word both are nouns.

Word	Noun	Adjective	Verb	Adverb
headword	Yes	No	No	No
head word	Yes	No	No	No

As nouns, headword and head word are synonyms defined as:

headword and head word: a content word that can be qualified by a modifier

headword (noun)	head word (noun)
a word placed at the beginning of a line or paragraph (as in a dictionary entry)	(grammar) the word in a grammatical constituent that plays the same grammatical role as the whole constituent
a content word that can be qualified by a modifier	a content word that can be qualified by a modifier