flight of stairs vs staircase

flight of stairs and staircase both are nouns.

Word	Noun	Adjective	Verb	Adverb
flight of stairs	Yes	No	No	No
staircase	Yes	No	No	No

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

flight of stairs: a stairway (set of steps) between one floor or landing and the next
staircase: a way of access (upward and downward) consisting of a set of steps

Other hypernyms of flight of stairs include stairway.

flight of stairs (noun)	staircase (noun)
a stairway (set of steps) between one floor or landing and the next	a way of access (upward and downward) consisting of a set of steps