retention vs possession

retention and possession both are nouns.

Word	Noun	Adjective	Verb	Adverb
retention	Yes	No	No	No
possession	Yes	No	No	No

As nouns, possession is a hypernym of retention; that is, possession is a word with a broader meaning than retention:

Other hypernyms of retention include ownership.

retention (noun)	possession (noun)
the act of retaining something	anything owned or possessed
the power of retaining liquid	the act of having and controlling property
the power of retaining and recalling past experience	(sport) the act of controlling the ball (or puck)
	the trait of resolutely controlling your own behavior
	a territory that is controlled by a ruling state
	a mania restricted to one thing or idea
	being controlled by passion or the supernatural