Is there a word for a concept defined by the lack of something?

@Matt Ellen used the word privative in this sense in his (closed) question, Is the truth a privative?

He used the word as a noun and the link from his question to the word's definition at Dictionary.com does have this:

privative
-noun something that is deprived.

I'm not sure how common this usage is. NOAD has this:

privative |ˈpraɪvədɪv|
adjective
(of an action or state) marked by the absence, removal, or loss of some quality or attribute that is normally present.
• (of a statement or term) denoting the absence or loss of an attribute or quality : the wording of the privative clause.
• Grammar (of a particle or affix) expressing absence or negation, for example, the a- (from the alpha privative in Greek), meaning “not,” in atypical.

noun
a privative attribute, quality, or proposition.

I would describe this as a "negative concept", or "negative definition", in the same fashion (and by analogy with) a negative right.

Via negativa might be the word(s) you're looking for. A quick Google search brings up: "a way of describing something by saying what it is not." It is from Latin, normally used to describe God. e.g. infinite is a word or concept which denies that the finiteness of something. Per your example, cold is a concept used to describe temperature via negativa, by negating the presence of heat.