The degree of combining power of an atom (or radical) as shown by the number of atoms of hydrogen (or of other monads, as chlorine, sodium, etc.) with which it will combine, or for which it can be substituted, or with which it can be compared; thus, an atom of hydrogen is a monad, and has a valence of one; the atoms of oxygen, nitrogen, and carbon are respectively dyads, triads, and tetrads, and have a valence respectively of two, three, and four.

measure of the number of antigens contained in a vaccine.

A small positive or negative whole number, also called oxidation number, which indicates the net number of electrons gained or lost in the formation of an ion, or the number of electrons the substance can donate or accept in a chemical reaction, and thus the numbers of each kind of ion necessary for a balanced chemical reaction. For example, two hydrogen ions (each with a valence of +1) must be present for each ion of oxygen (-2) to form a molecule of water (H2O).

a signed integer describing the combining power of an atom.