If any straight line inside the body keeps the same direction during motion, then motion is called translation. In a translation all the particles forming the body move along parallel paths. If these paths are straight lines, the motion is said to be rectilinear translation and if the paths are curved lines, the motion is a curvilinear translation.
Fig. 3.1 (a) Rectilinear translation Fig. 3.1 (b) Curvilinear translation
In case of translation the motion of the body is Completely specified by the motion of any point in the body and hence all the rectilinear motion for particles is applicable for rigid body motion for translation.