The property of matter by which it remains in its state of rest or its velocity along a straight line so long until an external force act upon it.So we can say that 'Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion.'

Inertia depends on the mass of the object. The more mass of the object more the inertia.So a solid ball of aluminium will have the  highest inertia because it has the highest mass than the other balls of same shape and volume.