ahead vs onward

ahead is an adjective but onward is not an adjective.

Word	Noun	Adjective	Verb	Adverb
ahead	No	Yes	No	Yes
onward	No	No	No	Yes

As adverbs, ahead and onward are synonyms defined as:

Other synonyms of ahead include forrader, forward, forwards, onwards.

ahead (adjective)	onward (adjective)
having the leading position or higher score in a contest

ahead (adverb)	onward (adverb)
at or in the front	forward in time or order or degree
ahead of time; in anticipation	in a forward direction
in a forward direction
leading or ahead in a competition
to a different or a more advanced time (meaning advanced either toward the present or toward the future)
to a more advanced or advantageous position
toward the future; forward in time