prayer vs intercession

prayer and intercession both are nouns.

Word	Noun	Adjective	Verb	Adverb
prayer	Yes	No	No	No
intercession	Yes	No	No	No

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

prayer (noun)	intercession (noun)
the act of communicating with a deity (especially as a petition or in adoration or contrition or thanksgiving)	the act of intervening (as to mediate a dispute, etc.)
earnest or urgent request	a prayer to God on behalf of another person
a fixed text used in praying
reverent petition to a deity
someone who prays to God