What is an inertial reference frame?

Answer: A system in which Newton's Laws are fulfilled

An inertial reference system is a reference system in which the principle of inertia is fulfilled, which is one of Newton's laws:

"For a body to have acceleration, an external force must act on it"

In addition, the other Newton's laws of movement are fulfilled.

Therefore, the variation of the linear momentum of the system is equal to the actual forces on the system.