it is not a vector nor a scalar but a tensor quantity!In fact, there are some physical quantities which are neither scalars nor vectors, but a sort of 'hybrid' of the two. Physicists have placed them in a category called 'tensors'. Moment of inertia, pressure, stress, density, dielectric current, density, etc. are tensors.
It is a measure of how mass is distributed in a body with respect to a center of rotation. Higher inertia means mass is distributed further away from the center of rotation. Units are Mass x Distance^2. Higher inertia means more difficult to start, keep, or stop rotating, similar to mass for linear acceleration.