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any member of a small class of words found in many languages that are used as replacements or substitutes for nouns and noun phrases, and that have very general reference, as I, you, he, this, who, what. Pronouns are sometimes formally distinguished from nouns, as in English by the existence of special objective forms, as him for he or me for I, and by nonoccurrence with an article or adjective.
something that follows a noun.
Pro noun is used instead noun.
It is good to use noun once or twice in a sentence and many other times it would be good to use Pronoun in the place of noun.
Think that, the President of USA Obama is addressing the nation. and in his speech he is repeating the USA so many times that it will start getting boring.
Instead if he used once USA and other times in his speech he substitute USA with THIS NATION, or THIS COUNTRY or OUR COUNTRY etc. It sound good and pleasing to the ear too.