I put the words "binary opposition" in the yahoo search engine and got this.
In critical theory, a binary opposition (also binary system) is a pair of related terms or concepts that are opposite in meaning. Binary opposition is the system by which, in language and thought, two theoretical opposites are strictly defined and set off against one another. It is the contrast between two mutually exclusive terms, such as on and off, up and down, left and right. Binary opposition is an important concept of structuralism, which sees such distinctions as fundamental to all language and thought  In structuralism, a binary opposition is seen as a fundamental organizer of human philosophy, culture, and language. In the community of philosophers and scholars, most believe that, as Derrida put it, "unless a distinction can be made rigorous and precise it isn't really a distinction."
The cultural studies theory known as Structuralism uses a term of art called "binary opposition" to explain human knowledge and to explain how many naturally occurring phenomena are constructed. Systems are "binary" when they are composed of only two parts. It's easy to imagine things "in opposition," like the Boston Red Sox and the New York Yankees, or the World War II alliances known as the "Axis Powers" and "the Allies." For an opposition to be truly "binary," however, the opposing classes of thing/idea must be mutually exclusive. That is, membership in one class must make impossible membership in the other. The baseball teams might be thought to be binary opposites, but remember that Babe Ruth played on both teams (hence the fabled "curse of the Bambino" on the Red Sox for trading him to the Yankees). If we are using the teams to construct the system of "major league baseball" for all time, it's apparent that one can play for both teams without destroying the system or one's self. The situation in World War II also resembled a binary opposition, with membership in the Axis or Allies being sharply distinguished in most cases, but some nations belonged to neither side, like the neutral states of Switzerland, Portugal, and Spain.