What describes motion in a straight line at constant speed?

Motion in a straight line at constant speed is best described as constant velocity. Velocity is a vector quantity that includes both the speed of the object and its direction of motion. When an object moves in a straight line and maintains the same speed without changing direction, it experiences constant velocity. This means that there are no changes in either the magnitude of the speed or the direction of motion; hence, the object's velocity remains unchanged throughout its motion.

In contrast, uniform acceleration refers to a situation where an object's velocity changes at a constant rate, which does not apply when speed is constant. Variable motion involves changes in velocity, indicating that speed or direction is varying, which again does not align with the concept of constant speed. Accelerated motion specifically indicates that there is a change in velocity, which is inconsistent with motion at constant speed.

Thus, constant velocity is the precise term that captures the notion of an object moving in a straight line at an unchanging speed.