[Text available only in English] A dichotomy is any splitting of a whole into exactly two non-overlapping parts, meaning it is a procedure in which a whole is divided into two parts, or in half. It is a partition of a whole (or a set) into two parts (subsets) that are:

- Jointly exhaustive: everything must belong to one part or the other, and

- Mutually exclusive: nothing can belong simultaneously to both parts.

The two parts thus formed are complements. In logic, the partitions are opposites if there exists a proposition such that it holds over one and not the other.

from: http://en.wikipedia.org/wiki/Dichotomy