An exotic atom is an atom, which is to say a system of particles held together primarily by the force due to electric charge, containing one or more unusual constituents. Electrons may be replaced by other negative particles, e.g. a muon or a pi-minus, or the positively charged nucleus can be replaced by other positively charged elementary particles.
strange matter, a hypothetical form of ultra-dense matter in which a large number of strange quarks are bound together with an electron "atmosphere" around them.
