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 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.