One of the fundamental concepts of mechanics is that of a particle. By this we mean a body whose dimensions may be neglected in describing its motion. The possibility of so doing depends, of course, on the conditions of the problem concerned. For example, the planets may be regarded as particles in considering their motion about the Sun, but not in considering their rotation about their axes.
Lev Landau and Evgeny Lifshitz, Mechanics (3rd ed., 1976), Chap. 1 : The Equations of Motion