A bound state consisting of an electron and a positron. There are two types of positronium: orthopositronium, in which the spins of the two constituents are parallel, and parapositronium, in which the spins are antiparallel. Both forms have brief existences, with orthopositronium decaying into three photons in about 1.5×10−7 s and parapositronium decaying into two photons in the even shorter time of about 10−10 s. Positronium has a hydrogen-like spectrum, but with different values of the frequencies since a positron is much lighter than a proton. Calculations on positronium provide good tests of quantum electrodynamics.