Protonium is a type of exotic atom in which an antiproton orbits a proton or a proton orbits an antiproton. Because it contains both matter and antimatter, protonium is called a type of "mass" instead of a type of "matter."
These exotic atoms are made in particle accelerator laboratories by making an antiproton come to rest near a proton, thus causing one to orbit the other. The result is an exotic atom that exists for a fraction of a second before the particles annihilate each other. This form of mass allows scientists to directly measure the mass of antiprotons, which are very close to that of protons. The masses are quite similar, but it is unknown if they are exactly the same.
Protonium is named after the subatomic particles which comprise it, similar to neutronium, which is composed of neutrons. Similarly, there is pionium, muonium, and other forms of mass that are made of very exotic particles.
Protonium is not known to occur naturally.