noun

1. the capacity of an element or radical to combine with another to form molecules, as measured by the number of hydrogen or chlorine atoms which one radical or one atom of the element will combine with or replace (e.g.: oxygen has a valence of two, i.e., one atom of oxygen combines with two hydrogen atoms to form the water molecule, HO)
2. any of the units of valence which a particular element may have

Etymology: LL valentia, worth, capacity < L, vigor < valens, prp. of valere, to be strong: see value