On the Newtonian system the motion of a particle entirely uninfluenced by other bodies, when referred to a suitable base, would be rectilinear, with constant velocity.
a point moving with a constant velocity having the magnitude and direction.
In the motion of a projectile under gravity the hodograph is a vertical line described with constant velocity.
In the case of a particle oscillating under gravity on a smooth cycloid from rest at the cusp the hotlograph is a circle through the pole, described with constant velocity.
For example, the mass-centre of I system free from extraneous force will describe a straight lin with constant velocity.