combustion vs ignition

combustion and ignition both are nouns.

Word	Noun	Adjective	Verb	Adverb
combustion	Yes	No	No	No
ignition	Yes	No	No	No

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

Other hyponyms of combustion include arson, fire-raising, incendiarism, firing, inflammation, kindling, lighting, incineration.

combustion (noun)	ignition (noun)
the act of burning something	the act of setting something on fire
a process in which a substance reacts with oxygen to give heat and light	the mechanism that ignites the fuel in an internal-combustion engine
a state of violent disturbance and excitement	the process of initiating combustion or catching fire