head off vs forestall

head off and forestall both are verbs.

Word	Noun	Adjective	Verb	Adverb
head off	No	No	Yes	No
forestall	No	No	Yes	No

As verbs, forestall is a hypernym of head off; that is, forestall is a word with a broader meaning than head off:

head off: prevent the occurrence of; prevent from happening; to protect from or to keep away anything undesirable; to ward off
forestall: keep from happening or arising; make impossible

Other hypernyms of head off include forbid, foreclose, preclude, prevent.

head off (verb)	forestall (verb)
prevent the occurrence of; prevent from happening; to protect from or to keep away anything undesirable; to ward off	act in advance of; deal with ahead of time
	keep from happening or arising; make impossible