An ion with a negative charge.

Flexable is mostly correct but it is wrong to say that all anions are always nonmetallic.
Anions do not have to be simple, i.e. they can be composed of a number of atoms, e.g. chromate (CrO₄^2-) and in this way it would be incorrect to say that they are either metallic or nonmetallic.
Sorry to be a pedantic asshole but I guess that's what science is all about.

An"i*on (#), n. [Gr. , neut. , p. pr. of to go up; up + to go.] Chem.

An electro-negative element, or the element which, in electro-chemical decompositions, is evolved at the anode; -- opposed to cation.

Faraday.

© Webster 1913.