A Lacanian concept derived from Saussurean structural linguistics. In structural linguistics, language is a a system in which there are no positive terms, only differences. This means that language only refers to language; that words are only distinct because they are not other words. Imagine looking for a definition of a word in a dictionary. When one finds the definition it consists of only other words. This endless chain of signifiers is halted by the master-signifier.
The master-signifier is a signifier that points to itself instead of other signifiers. Žižek refers to Marx's conception of commodity fetishism as an example of a master-signifier:
Money refers to value as such, and all other commodities are thought of in terms of how much money one can get for them. That is, money as a commodity becomes self-referential -- money is worth (signifies) money, instead of being worth X number of commodities -- and all other commodities are worth (signify) money.
Just as money in Marx's conception of commodity fetishism is in-itself devoid of value, the master-signifier is devoid of value, but provides a Point de capiton or quilting point around which other signifiers can stabilize.
- Kotsko, Adam, Žižek and Theology, 2008, 30.