Suppose the particles of a rigid body follow the same direction during motion. This type of motion is termed as translation. If all these particles are moving along parallel paths, then this type of translation is termed as rectilinear translation. And, if all these particles are moving along curved paths, then this type of translation is termed as curvilinear translation.
Suppose the translation of motion is determined by motion of any point in the rigid body. In this condition, the motion of particles will be considered as rectilinear for the rigid body.