Positronium is a quasi-stable system consisting of a positron and an electron bound together into an "exotic atom." Its set of energy levels is similar to that of the hydrogen atom (electron and proton). However, because of the different reduced mass, the frequencies associated with the spectral lines are less than half of those of the corresponding hydrogen lines. The half-life of positronium is at most about 10⁻⁷ seconds, its existence being terminated by electron-positron annihilation.

Positronium, like hydrogen, can have different configurations; the ground state can be a singlet state with antiparallel spins (S=0, M_s=0) known as parapositronium (term symbol ¹S₀). The other case, a triplet state, with spins parallel (S=1, M_s=−1,0,1) is known as orthopositronium (term symbol ³S₁).