understand vs grasp

understand is not a noun while grasp is a noun.

Word	Noun	Adjective	Verb	Adverb
understand	No	No	Yes	No
grasp	Yes	No	Yes	No

As verbs, grasp is a hyponym of understand; that is, grasp is a word with a more specific, narrower meaning than understand:

understand (noun)	grasp (noun)
	the act of grasping
	an intellectual hold or understanding
	understanding of the nature or meaning or quality or magnitude of something
	the limit of capability

understand (verb)	grasp (verb)
know and comprehend the nature or meaning of	get the meaning of something
perceive (an idea or situation) mentally	hold firmly
believe to be the case
make sense of a language
be understanding of