grasp vs hold on

grasp is a noun but hold on is not a noun.

Word	Noun	Adjective	Verb	Adverb
grasp	Yes	No	Yes	No
hold on	No	No	Yes	No

As verbs, grasp and hold on are synonyms defined as:

grasp (noun)	hold on (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

grasp (verb)	hold on (verb)
get the meaning of something	be persistent, refuse to stop
hold firmly	stop and wait, as if awaiting further instructions or developments
	hold the phone line open
	hold firmly
	retain possession of