Fifth-column definitions

A clandestine subversive organization working within a country to further an invading enemy's military and political aims.
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A group of people who aid the enemy from within their own country.
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(dated) A group of people which clandestinely undermines a larger group, such as a nation, to which it is expected to be loyal.
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Origin of fifth-column

From a 1936 radio address by Spanish Nationalist general Emilio Mola, in which he spoke of four of his army columns moving on Madrid and a quinta columna consisting of his militant supporters within the capital, intent on undermining the Republican government from within.