  The concept of inertial frame of reference in classical physics and special
theory of relativity 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.
