limb vs crus

limb and crus both are nouns.

Word	Noun	Adjective	Verb	Adverb
limb	Yes	No	No	No
crus	Yes	No	No	No

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

limb: one of the jointed appendages of an animal used for locomotion or grasping: arm, leg, wing, flipper
crus: the leg from the knee to foot

Other hyponyms of limb include hind limb, hindlimb, forelimb, flipper, leg, leg, thigh, arm, cubitus, forearm.

limb (noun)	crus (noun)
any projection that is thought to resemble a human arm	the leg from the knee to foot
the graduated arc that is attached to an instrument for measuring angles
either of the two halves of a bow from handle to tip
one of the jointed appendages of an animal used for locomotion or grasping: arm, leg, wing, flipper
(astronomy) the circumferential edge of the apparent disc of the sun or the moon or a planet
any of the main branches arising from the trunk or a bough of a tree