What is "exotic atom"

n. (context physics chemistry English) the equivalent of an atom in which either the nucleus, or an electron, is replaced by some other subatomic particle

An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes.