why 
(wī, hwī)
(wī, hwī)adv.
For what purpose, reason, or cause; with what intention, justification, or motive: Why is the door shut? Why do birds sing?
conj.
1. The reason, cause, or purpose for which: I know why you left.
2. Usage Problem On account of which; for which.
n. pl. whys
1. The cause or intention underlying a given action or situation: studying the whys of antisocial behavior.
2. A difficult problem or question.
interj.
Used to express mild surprise, indignation, or impatience.
[Middle English, from Old English hwȳ; see kwo- in the Appendix of Indo-European roots.]
Usage Note: A traditional rule states that why is redundant in the expression reason why, as in The reason why he accepted the nomination is not clear. It is true that why could be eliminated from such examples with no loss to the meaning, and that that could be used instead of why, but reason why has been used by reputable English writers since the Renaissance, and a majority of the Usage Panel accepts the construction. In our 2009 survey, 55 percent accepted the example quoted above. Higher percentages accepted examples in which reason is modified by a, any, or another instead of the.. For instance, 67 percent accepted the example Is there any reason why this drug enforcement program should work? While there is certainly no harm in observing this rule in one's own writing, insisting on it in the writing of others may seem petty. See Usage Note at where.