Doppler ultrasound commonly measures the flow velocity of blood.
Doppler ultrasound sends high-frequency sound waves into the body and analyzes the frequency shift in echoes returning from moving red blood cells. If blood moves toward the probe, the detected frequency increases; if it moves away, the frequency decreases. The size of that shift helps calculate the speed and direction of blood flow.
This technique is especially useful in cardiovascular medicine. It can examine arteries and veins, assess narrowing or blockage, detect abnormal flow, and evaluate jets of blood passing through heart valves. Color Doppler displays movement and direction over a conventional brightness-mode ultrasound image, while spectral Doppler can show velocity over time as a waveform.
The question’s wording says “commonly,” because Doppler ultrasound can also study other moving tissues and body fluids. Nevertheless, blood is the standard and canonical answer in medical imaging. Lymph, saliva, and bile may be medically relevant fluids, but they are not the usual target of routine Doppler ultrasonography.
Doppler ultrasound is noninvasive and does not use ionizing radiation. Its measurements depend on the angle between the ultrasound beam and the direction of flow, so sonographers try to keep that angle suitable for reliable velocity estimates.