  The concept of inertial frame of reference is analysed. It has been shown
that this fundamental concept of physics is not clear enough. A definition of
inertial frame of reference is proposed which expresses its key inherent
property. The definition is operational and powerful. Many other properties of
inertial frames follow from the definition or it makes them plausible. In
particular, the definition shows why physical laws obey space and time
symmetries and the principle of relativity, it resolves the problem of clock
synchronization and the role of light in it, as well as the problem of the
geometry of inertial frames.
