Definition: Valence is typically, the number of electrons needed to fill the outermost shell of an atom. Because exceptions exist, the more general definition of valence is the number of electrons with which a given atom generally bonds or number of bonds an atom forms. (Think iron, which may have a valence of 2 or a valence of 3.)
Examples: A neutral carbon atom has 6 electrons, with an electron shell configuration of 1s22s22p2. Carbon has a valence of 4, since 4 electrons can be accepted to fill the 2p orbital.