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Definition
When something is moving at a constant speed, it means that it is currently traveling at a steady pace without any acceleration or deceleration. This is a slightly different distinction than saying that something has a permanent, constant speed. If something has a permanent, constant speed, it reaches its full speed instantaneously without any initial acceleration and will never decelerate or stop.
Constant Speed vs. Constant Velocity
Although people often use the words "speed" and "velocity" interchangeably in everyday conversation, they mean very different things to physicists. Speed is a measure of distance traveled over time, while velocity is a measure of distance traveled over time in a specific direction. Do not confuse the difference between constant speed and constant velocity. If something is moving at a constant speed, it is moving at the same pace in any or all directions. If something is moving at a constant velocity, it is moving at the same pace in one direction.
Theoretical Example
A car is travelling on the highway at 75 mph. The driver has turned on cruise control and has not used the brake at all. The car and the driver inside could be said to be moving at a constant speed. However, neither the car nor the driver has a permanant, constant speed. Not only did they have to intially accelerate, they will eventually slow down and stop.
Naturally Occurring Example
Light has a permanent, constant speed of 299,792,458 m/s when travelling through a vacuum. When produced, light instantaneously moves at this rate and never decelerates. The constant speed of light is the foundation for many scientific theories in physics. However, the speed of light is subject to change once it leaves the vacuum of space and enters the atmosphere of a celestial body.