Illustrated Glossary of Organic Chemistry

Valence: The number of covalent bonds possessed by an atom.

In methanol (CH3OH) carbon has a valence of four (it is tetravalent), oxygen has a valence of two (it is divalent), and hydrogen has a valence of one (it is monovalent).

Methyl carbocation
Methyl carbanion
The methyl carbocation and methyl carbanion both have a carbon with valence of three (it is trivalent).

Carbon with a valence of five (a pentavalent carbon, also called a texas carbon) is an organic chemistry no-no.