Moment of inertia is:

actually its a tensor quantity but in some books it is given as scalar quantity. Tensor quantity means sometimes a parameter or a quantity behaves as scalar and sometimes it behaves as vector. Here Inertia is the property of mass. And moment of inertia occurs when body is in rotational motion, so if you consider one spherical object which is rotating along its diameter as axis of rotation you will notice that moment of I ertia depends on the distance of each and every particle from the axis of rotation. Hence sometimes it depends on the directions and sometimes depends on the distribution of mass of the particles in the sphere. Hence it's a tensor quantity combination of both scalars and vectors.