Definition of what:
(n) :
which thing, event, circumstance, etc.: used interrogatively in asking for the specification of an identity, quantity, quality, etc;
that;
which
"what is your address?"
(v) :
in some manner or degree; in part; partly; usually followed by with; such; this is; that is
"what with singing and joking, the time passed quickly."
(conjunction) :
that which; those which; the thing that
"he knows what he wants."
(interjection) :
an expression of surprise or disbelief; [mostly British colloquialism]: is that not true?
"what! That's amazing."