possession vs retention

possession and retention both are nouns.

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

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

Other hyponyms of possession include actual possession, constructive possession, criminal possession, holding, keeping.

possession (noun)	retention (noun)
anything owned or possessed	the act of retaining something
the act of having and controlling property	the power of retaining liquid
(sport) the act of controlling the ball (or puck)	the power of retaining and recalling past experience
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