The theta meson (θ) is a hypothetical form of quarkonium (i.e. a flavourless meson) formed by a top quark (t) and top antiquark (t). As a P-odd and C-odd t t state, it is analogous to the ϕ (s s), J/ψ (c c) and ϒ (b b) mesons. Due to the top quark's short lifetime, the theta meson is expected to not be observed in nature.