In daily discussions with friends, a theory and a hypothesis are no different. However, in science, a theory is a synthesis of a huge body of information and experimentation that support the suppositions made in the theory. If one piece of verifyable and repeatable evidence comes along that dissproves the theory, the theory is changed. If an object's speed is 50 miles per hour, its velocity could be 50 miles per hour, to the right. For velocity, direction must be included.
Constant velocity means the object is maintaining a constant speed and direction.