a noun denoting something that is countable: it can form a plural and be preceded by a or an or a numeral (Ex.: dog, tree, idea)
See count noun in American Heritage Dictionary 4
noun
A noun for an object, such as chair, or for an idea, such as experience, that speakers of a language identify as referring to a single entity and that can form a plural or occur in a noun phrase construction with an indefinite article, with numerals, or with such terms as many.