"We were not yet prepared to claim that we had found a new charged lepton, but we were ready to claim that we had found something new. To accentuate our uncertainty I denoted the new particle by U for unknown in some of our 1975-1977 papers. The name came later. This name was suggested by Rapidis, who was then a graduate student and had worked with me in the early 1970s on the problem. The letter is from the Greek for "third" -the third charged lepton".

Martin Perl, The Discovery of the Tau Lepton, in "The Rise of the Standard Model", Cambridge Univ. Press 1997.