The rest frame of a massive particle is an inertial frame in which its spatial momentum is zero. Its four-momentum is then and its energy is . Such a frame exists for a future-directed timelike four-momentum: the Lorentz transformation with boost velocity makes the spatial momentum zero, and the mass shell relation implies . A massless particle has no rest frame, because would require a boost with speed .
An inertial frame in which a selected body's velocity vanishes at one event on its worldline. For an accelerated body it is a local construction: rest frames at different events need not coincide, and simultaneity slices chosen at one event do not equal laboratory equal-time slices.
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In physics, particularly in the context of relativity, the term "rest frame" refers to a reference frame in which an object is at rest. In this frame, the object is not moving relative to the observer, and all measurements (like distance, time, and energy) can be made without the complications introduced by motion. For example, consider a person standing still while observing a car that is moving past them.