The term 'PARTICLE IN A CLOUD CHAMBER' can be broken down into two parts: the particle and the cloud chamber. The particle is an ION, which is an atom or molecule that has gained or lost one or more electrons, giving it a net electric charge. The cloud chamber is a device used to detect ionizing radiation, such as alpha particles, beta particles, and gamma rays. It works by creating a supersaturated vapor that condenses around the ionizing particles, forming visible trails that can be photographed and studied.