Matter
Anything that possesses mass, occupies volume, and is composed of fundamental particles such as quarks and leptons.
Governing formula
E = mc²
SI unit
kg, m³
In depth
Matter exists in four fundamental states: solid, liquid, gas, and plasma (along with exotic states like Bose-Einstein condensates). Ordinary matter consists of atoms made of protons, neutrons, and electrons.
Examples in the real world
Ice (solid water), liquid mercury, interstellar gas clouds, and high-temperature fusion plasma.