An object that is treated as having no mass.{. Photons are massless objects, having no rest mass, but in this case we are generally speaking of real, tangible objects whose mass we ignore, generally because their masses are so small compared to that of other elements of the system that ignoring their mass does not significant affect the results of our calculations. This is thus, in a sense, the opposite of an infinitely massive object. It is just as unreal but just as useful, pedagogically.
For instance, we often ignore the mass of the spring in the classic system of a mass on a spring, since this makes the complete equations of motion complicated and obscures the essential point. Similarly, the simplest model of a simple pendulum ignores the mass of the supporting string or rod, again, to simplify the mathematics and make the essential physics more clear.