Fluid Mechanics:
The rate at which the velocity changes with the distance across the flow.
When a fluid flows past a stationary wall, the
fluid right close to the wall does not move. However, away from the
wall the flow speed is not zero. Therefore a velocity gradient exists, which is
due to adhesive, cohesive, and frictional forces. The
amount of the velocity gradient is characteristic of the fluid.

In the theory of → radiation-driven winds,
an equation that describes the behavior of the → wind velocity
of → hot stars as a function of distance from the star.
This velocity β-law is given by the expression:
v(r) = v_{∞}(1 - R_{*}/r)^{β},
where v_{∞} is the → terminal velocity,
R_{*} is the stellar radius, and r the distance from the center.
For → O-type stars, the exponent is estimated to be β = 0.8.

A → physical constant which represents the ultimate speed limit
for anything moving through space, according to the theory of
→ special relativity.
It is the speed of propagation of → electromagnetic waves
in a vacuum, equal to 299,792.458 km/s (nearly 3 x 10^{8} m/s).
The velocity of light appears as the connecting link between mass and energy
in the → mass-energy relation.
Usually denoted by c, from L. celeritas "swiftness," from
celer "swift," → acceleration.

The linear relation wherein all galaxies are moving away from one another,
with velocities that are greater with increasing distance of the galaxy.
Same as → Hubble's law.

In fluid mechanics, a measure of the rate of rotational spin in a fluid.
Mathematically, vorticity is a vector field defined as the curl of the velocity field:
ω = ∇ x v. Meteo.: The rotation of air around a vertical axis.

In the → restricted three-body problem,
a surface which limits the region of space in which a small
body can move.
In the expression for the → Jacobi integral,
the left side value is always positive or nul; hence the particle motion
is confined to the region where U ≤ C_{J}. The surface that
limits this region, defined by U = C_{J}, is
called the zero-velocity surface.