Counter-jihad, also known as the counter-jihad movement, is a self-titled political current loosely consisting of authors, bloggers, think tanks, street movements and so on linked by beliefs that view Islam not as a religion but as an ideology that constitutes an existential threat to Western civilization. Consequently, counter-jihadists consider all Muslims as a potential threat, especially when they are already living within Western boundaries. Western Muslims accordingly are portrayed as a "fifth column", collectively seeking to destabilize Western nations' identity and values for the benefit of an international Islamic movement intent on the establishment of a caliphate in Western countries. The counter-jihad movement has been variously described as anti-Islamic, Islamophobic, inciting hatred against Muslims, and far-right. Influential figures in the movement include the bloggers Pamela Geller and Robert Spencer in the US, and Geert Wilders and Tommy Robinson in Europe.

A fifth column is any group of people who undermine a larger group or nation from within, usually in favor of an enemy group or another nation. According to Harris Mylonas and Scott Radnitz, "fifth columns" are "domestic actors who work to undermine the national interest, in cooperation with external rivals of the state". The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, espionage, and/or terrorism executed within defense lines by secret sympathizers with an external force.