What is it called when something is described by what it isn't?

I'm not looking for a particular part of speech.

A corollary to this idea, and in some ways the opposite, is when something is defined by/as itself. A lot of times you will find this in bad dictionaries when the definition of a word refers to a word whose definition refers to the original word. I am wondering if there is a word to describe this phenomenon.

I also recognize that using 'whose' above is grammatically incorrect, as it refers to a person. What would I replace it with?

Solution 1:

Interesting question! Are you looking for a word for “negative definition”?

In the theory of logic something can be defined with “double negation”.

In philosophy, Hegel helps with antithesis ‒ the opposite of thesis ‒ to explain ideas.

Solution 2:

I think what you are looking for is "negative definition". It is often used where it is much easier to contrast something with what it isn't than define exactly what it is. This gives an example (rather simplistic but...).

Solution 3:

Theologically, the term used for definition through negation is apophatic.

Definition:

(Of knowledge of God) Obtained through negation. The opposite of cataphatic.

Example:

Eastern Orthodox Christianity emphasizes apophatic theology - the Western equivalent is sometimes called ‘negative’ theology.

(Oxford Dictionaries Online)