A reference frame in which Newton's laws are valid; a non accelerating reference frame.

1. Frames in which Newton's laws are valid.
2. Any point of view whose velocity is a constant. A person in a car going down I5 would be an example of an inertial reference frame as the velocity would not be changing. However, going around a loop-de-loop would not be an inertial reference frame as the direction of the velocity is changing.