What is a definition of a compound noun?

A compound noun is a noun made up of two or more Words merged into one Word with a meaning of its own. There are three types of compound nouns:

open spaced

compound noun

tennis shoe

front door

paint brush

pork chop

mouth wash

hyphenated

mother-in-law

fifty-five

six-pack

yo-yo

T-shirt

free-for-all

closed

bathtub

Baseball

houseboat

watermelon

sidewalk

rainbow