meal vs oatmeal

meal and oatmeal both are nouns.

Word	Noun	Adjective	Verb	Adverb
meal	Yes	No	No	No
oatmeal	Yes	No	No	No

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

meal: coarsely ground foodstuff; especially seeds of various cereal grasses or pulse
oatmeal: meal made from rolled or ground oats

meal (noun)	oatmeal (noun)
the food served and eaten at one time	porridge made of rolled oats
coarsely ground foodstuff; especially seeds of various cereal grasses or pulse	meal made from rolled or ground oats
any of the occasions for eating food that occur by custom or habit at more or less fixed times