Question

What defines a nonmetal?

Answer 1

A non-metal is a substance that shows none of the properties we associate with metals. (not kidding here! read on...)

Explanation:

It would probably be best to first ask "What is a metal"?

A metal is a substance that has a certain collection of physical properties - good electrical and heat conductivity, malleable, ductile, shiny and reflective surface.

With that established, a non-metal is a material that does not demonstrate these properties at all, and a semi-metal (or metalloid) is one that shows some, but not all of the properties.

In my opinion, this is a better way to define a non-metal than trying to do so on the basis of valence electrons for example.