action vs piano action

action and piano action both are nouns.

Word	Noun	Adjective	Verb	Adverb
action	Yes	No	Yes	No
piano action	Yes	No	No	No

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

action: the operating part that transmits power to a mechanism
piano action: action consisting of a system of levers that move a felt hammer to strike the strings when a key is depressed

Other hyponyms of action include firing mechanism, gunlock, movement, pump action, slide action, action.

action (noun)	piano action (noun)
the most important or interesting work or activity in a specific area or field	action consisting of a system of levers that move a felt hammer to strike the strings when a key is depressed
a judicial proceeding brought by one party against another; one party prosecutes another for a wrong done or for protection of a right or for prevention of a wrong
something done (usually as opposed to something said)
a military engagement
an act by a government body or supranational organization
the operating part that transmits power to a mechanism
In firearms terminology, the mechanism that handles the ammunition (loads, locks, fires, and extracts the cartridges).
the trait of being active and energetic and forceful
the series of events that form a plot
a process existing in or produced by nature (rather than by the intent of human beings)
the state of being active

action (verb)	piano action (verb)
put in effect
institute legal proceedings against; file a suit against