A fifth-columnist is a person who works to undermine their own country from within, typically by aiding an enemy. This term is derived from the Spanish Civil War, when four columns of Nationalist forces marched on Madrid while a fifth column of sympathizers worked from within the city to support the Nationalists. In a crossword puzzle, the term 'fifth-columnist' can be represented by the solutions 'SPY' (3 letters) and 'TRAITOR' (7 letters).