An instrument that measures the velocity of a flowing liquid or gas, usually understood to be the average velocity of fluid within a relatively small measuring volume. Velocimeters are distinct from flow meters, which measure spatially averaged velocity, or flow rate, across the cross section of a duct or channel. Depending on the nature of the flow and the objective of the measurement, a velocimeter may be used to measure a time-dependent flow velocity or a time-averaged value. Another characteristic of a velocimeter is its spatial resolution, namely the size of its measuring volume. Some velocimeters measure flow velocity in both magnitude and direction, whereas others only resolve a velocity component in a particular direction or plane. The term anemometer is sometimes used as synonymous to velocimeter, although, strictly speaking, the former term should apply only to instruments that measure air velocity.