US and Doppler
Ultrasound is the use of ultrasonic sound to image tissue.
Doppler is an ultrasound term referring the Doppler effect described by HC Doppler, which uses sound waves to insonate a moving object, e.g. red blood cells in a vessel, to determine the direction of blood flow and flow velocity. Duplex uses B mode ultrasound to image tissue structure and Doppler to determine fluid flow.