What movement characteristic defines rectilinear motion?

Rectilinear motion is defined by movement along a straight line. This type of motion is characterized by the path taken by an object being linear, with no change in direction. It can occur at a constant speed or varying speeds, but the key aspect is the absence of any curvature in the trajectory; the object does not deviate from a straight path during its motion. In contrast, circular or curvilinear motions involve changes in direction and do not maintain a linear path. Therefore, the defining feature of rectilinear motion is that it strictly adheres to a straight line, making the correct answer the one that identifies this characteristic accurately.