The Bison was an improvised fighting vehicle frequently characterised as a mobile pillbox. Bisons were produced in Britain during the invasion crisis of 1940-1941. Based on a number of different lorry chassis, it featured a fighting compartment protected by a layer of concrete. Bisons were used by the Royal Air Force (RAF) to protect aerodromes and by the Home Guard. They acquired the generic name "Bison" from their main manufacturer.

An improvised fighting vehicle is an ad hoc combat vehicle resulting from modified or upgraded civilian or military non-combat vehicle, often constructed and employed by civilian insurgents, terrorists, rebels, mobsters, guerrillas, partisans, drug cartels, criminal organizations or other forms of non-state militias and irregular armies. Such modifications usually consist of grafting improvised armour plating and fixed crew-served weapons such as heavy machine guns or antiaircraft autocannons mounted onto the back of a utility vehicle or pickup truck.