Mu-meson, also known as the muon, is a subatomic particle with a negative charge and a mass of approximately 200 times that of an electron. It was first discovered in 1936, and since then, several synonyms have been used to refer to this particle. Some of the common synonyms include the mu-lepton, the mu meson, or simply the mu particle. Other less common synonyms include the middle electron, the heavy electron, or the muonic atom. Regardless of the synonym used to refer to this particle, it plays a significant role in particle physics research and has helped expand our understanding of the universe.