what

from The American Heritage® Dictionary of the English Language, 5th Edition.

• pronoun Which thing or which particular one of many.
• pronoun Which kind, character, or designation.
• pronoun One of how much value or significance.
• pronoun That which; the thing that.
• pronoun Whatever thing that.
• pronoun Informal Something.
• pronoun Nonstandard Which, who, or that.
• adjective Which one or ones of several or many.
• adjective Whatever.
• adjective How great; how astonishing.
• adverb How much; in what respect; how.
• conjunction That.
• interjection Used to express surprise, incredulity, or other strong and sudden excitement.
• interjection Chiefly British Used as a tag question, often to solicit agreement.
• idiom Informal (or what) Used as an intensive at the end of a question.
• idiom (what for) A scolding or strong reprimand.
• idiom (what for) For what reason; why.
• idiom (what have you) What remains and need not be mentioned.
• idiom (what if) What would occur if; suppose that.
• idiom (what if) What does it matter if.
• idiom (what it takes) The necessary expertise or qualities needed for success.
• idiom (what's what) The fundamentals and details of a situation or process; the true state or condition.
• idiom (what with) Taking into consideration; because of.

from The Century Dictionary.

• Why?
• To what degree? in what respect?
• How; how greatly; to what an extent or degree; how remarkably: exclamatory and intensive.
• With omission of the second what (so frequently):
• So much as; so far as.
• That
• In the phrase but what: but that; that … not.
• Quick; sharp; bold.
• Used absolutely as an interrogative pronoun.
• Applied to animals (and sometimes in contempt to persons) with the force of inquiry after the nature or kind: as, what is that running up the tree?
• Applied to persons: nearly equivalent to who, but having reference to origin or character, rather than to name or identity.
• Used in various elliptical and incomplete constructions: as, what? equivalent to what did you say? or what is it?
• Used in exclamation, to express surprise, indignation, etc.
• Expressing a summons.
• A general introductory notion, equivalent to ‘well,’ ‘lo,’ ‘now,’ etc., and constituting a mere expletive.
• Used adjectively and lending an interrogative force to the proposition in which it occurs.
• Inquiring as to extent or quantity: equivalent to the question how much?
• Used intensively or emphatically with a force varying from the interrogative to the exclamatory: often followed by the indefinite article: as, what an idea!
• Elliptical for what say or think you of?
• A compound relative pronoun, meaning ‘that which,’ or having a value including the simple relative pronoun which with the demonstrative pronoun that preceding: as, “what I have written I have written” (that is, that which I have written I have written). It is no longer used of persons, except in the anomalous phrase but what.
• What was formerly and in vulgar speech is still used as a simple relative, equivalent to that or which: as, if I had a donkey what wouldn't go.
• What has also the value of whatever or whoever: as, come what will, I shall be there.
• Used adjectively, meaning ‘that … which,’ or having compound relative value: as, I know what book you mean (that is, I know that book which you mean); he makes the most of what money he has (that is, he makes the most of that money which he has): applied to persons and things.
• What sort of; such … as.
• Any who or which; whatever; whoever.