An ultrasonic flow meter is a type of flow meter that measures the velocity of a fluid with ultrasound to calculate volume flow.  Using ultrasonic transducers, the flow meter can measure the average velocity.  It can also measure along the path of an emitted beam of ultrasound.

It does this by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow.  It also uses it to measure the frequency shift from the Doppler effect.

Also, Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter.  They vary greatly in purchase price but are often inexpensive to use and maintain.   This is because they do not use moving parts, unlike mechanical flow meters.