start vs embark on

start is a noun but embark on is not a noun.

Word	Noun	Adjective	Verb	Adverb
start	Yes	No	Yes	No
embark on	No	No	Yes	No

As verbs, embark on is a hyponym of start; that is, embark on is a word with a more specific, narrower meaning than start:

As verbs, start and embark on are synonyms defined as:

Other synonyms of start include commence, start up.

start (noun)	embark on (noun)
the act of starting something
a sudden involuntary movement
a turn to be a starter (in a game at the beginning)
the advantage gained by beginning early (as in a race)
a signal to begin (as in a race)
the beginning of anything
a line indicating the location of the start of a race or a game
the time at which something is supposed to begin

start (verb)	embark on (verb)
take the first step or steps in carrying out an action	get off the ground
set in motion, cause to start
play in the starting lineup
bring into being
get off the ground
begin or set in motion
get going or set in motion
depart for someplace
bulge outward
move or jump suddenly, as if in surprise or alarm
begin work or acting in a certain capacity, office or job
begin an event that is implied and limited by the nature or inherent function of the direct object
have a beginning, in a temporal, spatial, or evaluative sense
have a beginning characterized in some specified way