Define inertial frame of reference.

Explanation

### Step-by-Step Text Solution

1. **Definition of Inertial Frame of Reference**:
An inertial frame of reference is defined as a frame in which an object either remains at rest or moves at a constant velocity unless acted upon by an external force. This means that in an inertial frame, the laws of motion, particularly Newton's laws, hold true.

2. **Condition of Acceleration**:
In an inertial frame, the acceleration is zero. This implies that the frame is non-accelerated. If a frame is accelerating, it cannot be considered inertial because objects within that frame would appear to be influenced by fictitious forces.

...