Matter
Anything that occupies space and possesses rest mass, existing in the solid, liquid, gas or plasma state.
Unit
kg
In depth
Matter is built from atoms, which combine into molecules and extended lattices. Its two defining bulk quantities are mass and volume; everything else — colour, hardness, reactivity — emerges from how its constituent particles are arranged and bonded. Energy and fields are not matter, since they carry no rest mass.
Examples in the real world
A copper wire (solid), liquid mercury, nitrogen gas in the air, and the ionised gas inside a fluorescent tube are all matter.