A Countable Noun is a noun that has both a singular and a plural form. The plural is normally made by the addition of '-s'.
eg: A horse Two horses
Nouns that do not have plural forms are called uncountable nouns or mass nouns.
See Also: Concrete Noun; Proper Noun; Adjectival Noun; Noun as Adjective; Animate Noun; Inanimate Noun; Collective Noun; Substantive; Irregular Plurals