Answer: velocity
Step-by-step explanation:
Slope of velocity time graph = acceleration. A change in velocity = a change in acceleration. A constant velocity would mean that there would be no acceleration.
Speed and velocity aren't the exact same thing. Speed is a scalar measurement while velocity is a vector measurement. The speed of an object could be different from its velocity (ex: speed CANNOT be negative while velocity can).
An object can have a constant speed but have a changing velocity.