Moment of inertia is a measure of the rotational inertia of a body, which is its resistance to change in its angular velocity. It is defined as the sum of the products obtained by multiplying the mass of each particle of matter in a given body by the square of its distance from the axis of rotation. In simpler terms, it is the quantity expressed by the body resisting angular acceleration, which is the sum of the product of the mass of every particle with its square of the distance from the axis of rotation. The moment of inertia is also known as the angular mass or rotational inertia.

The moment of inertia is usually specified with respect to a chosen axis of rotation, and it mainly depends on the distribution of mass around an axis of rotation. The moment of inertia varies depending on the axis that is chosen. The SI unit of moment of inertia is kg m2.

The moment of inertia of a rigid body does not change with the speed of rotation. The moment of inertia of any body having a shape that can be described by a mathematical formula is commonly calculated by the integral calculus.

The moment of inertia is an important concept in rotational motion and is used in calculating angular momentum for a rigid body. For linear momentum, the momentum p is equal to the mass m times the velocity v; whereas for angular momentum, the angular momentum L is equal to the moment of inertia I times the angular velocity ω.

In summary, moment of inertia is a measure of the resistance of a body to change in its angular velocity, and it depends on the distribution of mass around an axis of rotation. It is usually specified with respect to a chosen axis of rotation and is an important concept in rotational motion.