Noun
pentaquark (plural pentaquarks)
(physics) Any of a class of subatomic particles (previously hypothetical, since detected, subject to confirmation) consisting of a group of five quarks (compared to three quarks in normal baryons and two in mesons), or more specifically four quarks and one antiquark (symbol Θ).
Understanding bound states of hadrons such as pionium and protonium is also important in order to clarify notions related to exotic hadrons such as mesonic molecules and pentaquark states. Source: Internet
While the status of tetraquarks is still under debate, all known pentaquark candidates have since been established as non-existent. Source: Internet