It is called a definition.
The definition of the speed of light in a vacuum is 300,000,000 meters/sec, or 186,000 miles/sec.
Initial speed is the original, beginning speed of an object.
they are fast
Quickness, speed
The speed that an object moves through the air.
Great speed is faster than normal.
"instantaneous speed"
By definition acceleration is the change in velocity (speed).
The definition of haste is: swiftness of motion; speed; celerity or unnecessarily quick action; thoughtless, rash, or undue speed
There are different types of speed formulas: Speed= Wavelength(frequency) or Speed= Distance/Time
The speed of light is not medium: in vacuum, it is the highest speed that there can be. Hardly a definition of medium!