head word vs headword

head word and headword both are nouns.

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

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

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

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