Definition of: why

(hwī) adverb
1. For what cause, purpose, or reason; wherefore: used interrogatively: Why did you go?
2. Because of which; for which; the reason or cause for which: used relatively: I don't know why he went; I know no reason why he went.
—noun plural whys
1. An explanatory cause; reason; cause.
2. A puzzling problem; riddle; enigma.
—interj. An introductory expletive, sometimes denoting surprise. [OE hwӯ, hwӯ, instrumental case of hwœt what]