1. (in physics) One of the fundamental components of matter. See elementary particles.

2. (in mechanics) A hypothetical body that has mass but no physical extension. As it is regarded as having no volume, a particle is incapable of rotation and therefore can only have translational motion. Thus a real body may often, for translational purposes, be regarded as a particle located at the body’s centre of mass and having a mass equal to that of the whole body.