Glossary of Literary Theory by Greig E. Henderson and Christopher Brown

Binary oppositions:

A structuralist term used to describe the differential nature of any signifying system. Binary oppositions are not facts or substances that have detectable positive qualities, but relational elements that are detectable only by virtue of their difference from other elements intrinsic to the system itself. Thus individual terms acquire meaning only by being cast in opposition to other terms within a system of arbitrary and conventional signs. (See also Structuralism.)