Meaning

noun (biology) a relative capacity to unite or react or interact as with antigens or a biological substrate

noun (chemistry) a property of atoms or radicals; their combining power given in terms of the number of hydrogen atoms (or the equivalent)

originEtymology 1, noun sense 1.1 (“combining capacity of an atom”) and etymology 1, noun sense 3 (“one-dimensional value assigned by a person to an object, situation, or state”) are borrowed from German Valenz + English -ence (suffix meaning ‘having the condition or state of’). Valenz is a clipping of Quantivalenz (“(archaic) valence in chemistry”), from English quantivalence, from Latin quantus (“how…

Etymology from Wiktionary, CC BY-SA 3.0