The term “mass spectrometry” is usually abbreviated as “MS.” It is confusing that “MS” is also used as an abbreviation for “mass spectrometer,” “mass spectroscopy,” “mass spectrograph,” “mass spectrum (spectra),” and “mass separation.” “Masu” in Japanese is an ambiguous word and originates from “mass” in English or “Mass” in German. In addition to the meaning of mass, however, “masu” is used as an abbreviation for “mass spectrometry,” “mass spectrometer,” “unified atomic mass unit,” and “a quantity of m/z= 1” by Japanese mass spectrometrists. Because of the ambiguity in the usage of “MS” and “masu,” some inappropriate terms, such as “peptide MS fingerprinting” for “peptide mass fingerprinting” and “TOF-mass” for “time-of-flight mass spectrometer,” were used by Japanese mass spectrometrists.