The maximum number of univalent atoms that may combine with an atom of the element in question.
So if the element is, for example, oxygen, then it combines with two univalent atoms to form a stable compound (
If the element is sodium, it combines with one univalent atom to form a stable compound (NaCl). Valence is therefore 1.
Oxidation number is a more useful concept though, as it allows for the distinction between positive and negative, and also for elements where more than one oxidation state is possible (e.g. transition metals).