The term "defeat device" refers to any mechanism, physical or otherwise, designed to defeat emmissions controls. It's not literally a standalone gadget. I assume the term is a holdover from the 1970s.

    A defeat device is an AECD “that reduces the effectiveness of the emission
    control system under conditions which may reasonably be expected to be
    encountered in normal vehicle operation and use. Motor vehicles equipped
    with defeat devices cannot be certified.”

And the AECD is defined as

    any element of design which senses temperature, vehicle speed,
    engine RPM, transmission gear, manifold vacuum or any other parameter for
    the purpose of activating, modulating, delaying or deactivating the
    operation of any part of the emission control system.

http://www.autocar.co.uk/car-news/industry/vw-emissions-scan...