companionway vs staircase

companionway and staircase both are nouns.

Word	Noun	Adjective	Verb	Adverb
companionway	Yes	No	No	No
staircase	Yes	No	No	No

As nouns, staircase is a hypernym of companionway; that is, staircase is a word with a broader meaning than companionway:

companionway: a stairway or ladder that leads from one deck to another on a ship
staircase: a way of access (upward and downward) consisting of a set of steps

Other hypernyms of companionway include stairway.

companionway (noun)	staircase (noun)
a stairway or ladder that leads from one deck to another on a ship	a way of access (upward and downward) consisting of a set of steps