Generally, the mass of a body is defined as the amount of matter contained in it. But we also know from relativity that "mass of a body increases as it approaches the speed of light". Going by the old school definition mentioned above, this really doesn't make sense. Like, how can the amount of matter increase?

So what I have come up with, is that mass can be defined as net force/acceleration for a given velocity with respect to an inertial frame. Can you please tell me if it is correct, and can you please help me improve this definition(assuming it's correct in the first place)