valon nopeus

Noun

 * 1) The speed of light in a medium.
 * 2) The speed of light in a vacuum.

Usage notes
Formerly was always written as two separate words. It is becoming customary to write the two words together, when one refers to the speed of light in a vacuum (which is a universal constant, marked with c and equivalent to 299,792,458 m/s), and separately, when one refers to the actual light speed in a given medium (v < c). The usage is rather confused and either of the two forms may refer to either of the meanings.