A noun is a part of speech that serves as a subject or object. More generally, we think of a noun as a person, place, thing, quality, or action.
The word noun is noun, since a word is a thing. Tricky examples of nouns include gerunds: studying looks like a verb (and it is when you say "I'm studying for a test") but it's often a noun instead, as when you say, "Studying is very important." Sometimes a noun is missing but implied: When you tell your friend to "Go jump in a lake," the noun is you, as in "Hey, you, go jump in a lake."