ictus vs convulsion

ictus and convulsion both are nouns.

Word	Noun	Adjective	Verb	Adverb
ictus	Yes	No	No	No
convulsion	Yes	No	No	No

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

Other hyponyms of ictus include focal seizure, raptus hemorrhagicus, absence, absence seizure.

ictus (noun)	convulsion (noun)
a sudden occurrence (or recurrence) of a disease	a physical disturbance such as an earthquake or upheaval
	violent uncontrollable contractions of muscles
	a sudden uncontrollable attack
	a violent disturbance