executioner vs headsman

executioner and headsman both are nouns.

Word	Noun	Adjective	Verb	Adverb
executioner	Yes	No	No	No
headsman	Yes	No	No	No

As nouns, headsman is a hyponym of executioner; that is, headsman is a word with a more specific, narrower meaning than executioner:

executioner: an official who inflicts capital punishment in pursuit of a warrant
headsman: an executioner who beheads the condemned person

Other hyponyms of executioner include electrocutioner, hangman, headman.

executioner (noun)	headsman (noun)
an official who inflicts capital punishment in pursuit of a warrant	an executioner who beheads the condemned person