In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. The term "inertia" refers to a direction through spacetime, and "frame" defines an area wherein the inertia is functionally the same for the relevant observers.
The inertial frame is a space-time coordinate system that neither rotates nor accelerates. Different inertial reference frames may have different origins at any given moment in time, and their origins may be moving at constant speed and direction relative to each other. A non-inertial frame of reference is a coordinate system which is accelerating. The transformation from one inertial frame of reference to another is done using Lorentz transformations, or, at speeds considerably below the speed of light, Galilean transformations.
The term is used in cases where relativity is considered, and a meaningful difference is drawn between the views of two observers with respect to their time and space intervals. From within the same inertial frame of reference, functions of relativity can be adequately replaced by classical mechanics. The above transfomations are applied for scaling upward from the local frame to larger frames, and for backwards verification of general theories back to the observable local frame.
The frame concept is at the core of the questions dealing how the apparent classical mechanics of particular inertial frame relates to the general theories of the universe and spacetime. By the very nature of human limits to a particular inertial frame, the various branches of physics are devoted to building functions that relate human observation to theoretical concepts of cosmology and quantum mechanics and the rules by which they interoperate to describe possible reference frames like our own.
Inertial frames of reference appear prominently in both Newtonian relativity and Einstein's special theory of relativity. In Newtonian mechanics, any mass viewed from an inertial reference frame will appear either to be stationary or to be moving at constant speed in a straight line, if and only if the sum of forces acting upon that mass is zero. (This is also known as Newton's first law of motion.)
Frames of reference are purely theoretical, because gravitational force (and thus acceleration) exists everywhere in the known universe. However, they may be approximated very well in intergalactic space, or to a lesser extent within the confines of a coasting spacecraft. If you can find an inertial frame of reference for a given situation, then it can always be transformed by a change of coordinates into one in which the observers do not move at all.