particle velocity

In ocean wave studies, the instantaneous velocity of a water particle undergoing orbital motion.

It has the scalar value

where T is the wave period, H the wave height, z the depth below still-water level, and L the wave length. At the crest, its direction is the same as the direction of progress of the wave; at the trough it is in the opposite direction.
See orbital velocity.

An aggregation of sufficiently many atoms or molecules that it can be assigned macroscopic properties such as volume, density, pressure, and temperature.