A bound state consisting of a quark and its own antiquark. Quarkonium is the analogue for quarks of positronium. Like positronium, quarkonium has spectral structure. Examples of quarkonium include charmonium (the psi particle), which consists of a charm quark and its antiquark, and bottomonium, which consists of a bottom quark and its antiquark. It is not possible for toponium, i.e. a bound state consisting of a top quark and its antiquark, to exist because the top quark decays before a bound state can form. The properties of quarkonium can be analysed using the standard model of elementary particles.