Anything that has mass and takes up space.

Here's why this is the best definition:

* Mass: Matter has mass, which is a measure of its inertia, its resistance to changes in motion. This means matter has weight and can be affected by gravity.

* Takes up space: Matter occupies space, meaning it has volume. It cannot share the same space as other matter.

This definition covers all forms of matter, from the smallest subatomic particles to the largest celestial bodies. It excludes things like light, sound, and heat, which are forms of energy and not matter.

However, it's important to remember that even this seemingly simple definition has nuances in the world of physics. For example, light can exhibit some properties of matter under certain conditions, which is why it gets tricky to define matter in a strict sense at the quantum level. But for our everyday understanding, this definition is more than adequate.