posture vs decubitus

posture and decubitus both are nouns.

Word	Noun	Adjective	Verb	Adverb
posture	Yes	No	Yes	No
decubitus	Yes	No	No	No

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

posture (noun)	decubitus (noun)
the arrangement of the body and its limbs	a reclining position (as in a bed)
characteristic way of bearing one's body
capability in terms of personnel and materiel that affect the capacity to fight a war
a rationalized mental attitude

posture (verb)	decubitus (verb)
assume a posture as for artistic purposes
behave affectedly or unnaturally in order to impress others