what

determiner

1. Which, especially which of an open-ended set of possibilities.

   “What colour are you going to use?”

2. Which.

   “I wonder what colour he is going to use.”

3. Any ... that; all ... that; whatever.

   “He seems to have lost what sense he had.”

4. Emphasises that something is noteworthy or remarkable in quality or degree, in either a good or bad way; may be used in combination with certain other determiners, especially 'a', less often 'some'.

   “This shows what beauty there is in nature.”

pronoun

1. Which thing, event, circumstance, etc.: used in asking for the specification of an identity, quantity, quality, etc.

   “Hammer in that nail —What with? I didn't bring my set of tools.”

2. That which; those that; the thing(s) that.

   “He knows what he wants and what to do to get it.”

3. Anything that; all that; whatever.

   “I will do what I can to help you.”

4. That; which; who.

   “Look! It's that bloke again what was 'ere yesterday!”

adverb

1. In what way; to what extent.

   “What does it matter?”

2. Used before a prepositional phrase to emphasise that something is taken into consideration as a cause or reason; usually used in combination with 'with' (see what with), and much less commonly with other prepositions.

interjection

1. An expression of surprise or disbelief.

   “What! That’s amazing!”

2. What do you want? An abrupt, usually unfriendly enquiry as to what a person desires.

   “What? I'm busy.”

3. Clipping of what do you say?, used as a type of tag question to emphasise a statement and invite agreement, often rhetorically.

   “It’s a nice day, what?”

noun

1. Something; thing; stuff.

2. The identity of a thing, as an answer to a question of what.

3. Something that is addressed by what, as opposed to a person, addressed by who.