An inertial frame of reference is a coordinate system in which Newton's first law holds true. In other words, if an object is not subjected to any external forces, it will either remain at rest or move with constant velocity in an inertial frame.
Related terms
Non-inertial Frame of Reference: This term refers to a coordinate system that is accelerating or rotating, where Newton's first law does not hold true.
Galilean Transformation: This term refers to mathematical equations used to convert measurements between different inertial frames of reference.