a Lacanian term (following the linguistics of Saussure); really just another signifier (i.e., something that organises discursive structures) but one which stops the slippage of the signified under the signifier and fixes meaning, thereby forming a stable symbolic order. i don't really know tbh. a platonic ideal of a concept like "freedom" or "health"?
The master signifier was not ‘empire’ (politically inadmissible except, chiefly, as a designation for the Soviet Union) but ‘security’.