Valence (also called "valency") is a property of verbs. It indicates the number of arguments a verb requires to make a clause (not counting optionally expressed non-core arguments). Valence is roughly equivalent to the traditional concept of transitivity.
(1) contains a verb with a valence of 1 (sometimes called univalent or intransitive), (2) a verb with valence of 2 (divalent or transitive), and (3) a verb with valence of 3 (trivalent or ditransitive).
- 1) Jones snored.
- 2) Smith bewildered the pedestrians.
- 3) Jones gave the cheese to the mouse.
Although the mouse in (3) looks superficially like an object of a preposition (and therefore an oblique or non-core argument), it is in fact a core argument (indirect object) of the verb give. We can tell because eliminating the argument makes the sentence ungramattical (4).
- 4) *Jones gave the cheese.
While valence is conceived as a property of verbs (and therefore is lexically determined), human languages have a wide variety of valence changing operations by which valence can be increased or decreased. Passive voice, for instance, is one example of an "operation" which decreases the valence of a given verb.
- wikipedia article on Valence: http://en.wikipedia.org/wiki/Valency_(linguistics)