Linear velocity is the velocity of the object travelling in a straight line. Mathematically, it is given by v = s/t where v is the velocity in m/s and s is the distance in meters ; t is the time taken in seconds.

A body is said to be in constant linear velocity if it is moving with a constant speed without changing the direction.  

When the body changes its direction, it is no longer said to be linear. So consider a toy car moving in the direction of north. As long as it moves in north direction, it is said to be linear. However, when it turns slightly towards east or west, it is no longer said to be linear due to change in direction.

Hence, in linear velocity, the object never changes direction.