complaint vs grievance

complaint and grievance both are nouns.

Word	Noun	Adjective	Verb	Adverb
complaint	Yes	No	No	No
grievance	Yes	No	No	No

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

complaint: an expression of grievance or resentment
grievance: a complaint about a (real or imaginary) wrong that causes resentment and is grounds for action

Other hyponyms of complaint include grumble, grumbling, murmur, murmuring, mutter, muttering, jeremiad, kvetch, pet peeve, whimper, whine.

complaint (noun)	grievance (noun)
an expression of grievance or resentment	a complaint about a (real or imaginary) wrong that causes resentment and is grounds for action
(formerly) a loud cry (or repeated cries) of pain or rage or sorrow	an allegation that something imposes an illegal obligation or denies some legal right or causes injustice
(civil law) the first pleading of the plaintiff setting out the facts on which the claim for relief is based	a resentment strong enough to justify retaliation
(criminal law) a pleading describing some wrong or offense
an often persistent bodily disorder or disease; a cause for complaining