"Constant speed" is speed maintained consistently over time. To achieve constant speed, an object has to cover an equal distance for equal intervals of time. The acceleration of the object is zero when it is traveling at a constant speed.
The formula for constant speed is the total distance traveled by the object divided by the time in which the object traveled that distance. Constant speed has consistency maintained in any direction, while constant velocity has consistency maintained in one direction. An example of an object moving at a constant speed is a satellite. A satellite and other objects in space have a constant speed but a variable velocity, because their direction changes.