Wikipedia
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.
Binary opposition originated in Saussurean structuralist theory. According to Ferdinand de Saussure, the binary opposition is the means by which the units of language have value or meaning; each unit is defined in reciprocal determination with another term, as in binary code. It is not a contradictory relation but a structural, complementary one. Saussure demonstrated that a sign's meaning is derived from its context (syntagmatic dimension) and the group (paradigm) to which it belongs. An example of this is that one cannot conceive of 'good' if we do not understand 'evil'.
Typically, one of the two opposites assumes a role of dominance over the other. The categorization of binary oppositions is "often value-laden and ethnocentric", with an illusory order and superficial meaning. Furthermore, Pieter Fourie discovers that binary oppositions have a deeper or second level of binaries that help to reinforce meaning. As an example, the concepts hero and villain involve secondary binaries: good/bad, handsome/ugly, liked/disliked, and so on.