Real Dictionary

why
adverb: You use why in questions when you ask about the reasons for something.

why in American English
for what reason, cause, or purpose? with what motive?
adverb: for what? for what reason, cause, or purpose?
conjunction: for what cause or reason
interjection: (used as an expression of surprise, hesitation, etc., or sometimes a mere expletive)
noun: a question concerning the cause or reason for which something is done, achieved, etc

why in British English
adverb: for what reason, purpose, or cause?
exclamation: an introductory expression of surprise, disagreement, indignation, etc
noun: the reason, purpose, or cause of something (esp in the phrase the whys and wherefores)
pronoun: for or because of which