CLASSICAL ENDING. There are in English many nouns whose SINGULAR/PLURAL contrasts derive from LATIN and GREEK, such as stimulus/stimuli (Latin: masculine), formula/formulae (Latin: feminine), memorandum/memoranda (Latin: neuter), phenomenon/phenomena (Greek: neuter). During the 16–19c, when writing was largely the concern of the classically educated, many such endings were retained as a matter of course. Some are universally used (radius/radii), some have become restricted to certain registers (formulae to scientific discourse, formulas gaining ground generally; indexes in books, indices in mathematics), and some have been considerably adapted (the singular agendum has disappeared and the former plural agenda has become a singular, with the non-classical plural agendas). Asymmetry is common: campus and ultimatum have the plurals campuses and ultimatums, not *campi and *ultimata, while desideratum and sanctum sanctorum have the plurals desiderata and sancta sanctorum, not *desideratums and sanctum sanctorums. Many would hesitate when choosing plurals for such words as arboretum and thesaurus (both classical and vernacular are possible).

Although contrasts such as Latin addendum/addenda and Greek criterion/criteria are maintained in academic and technical writing, bacterium/bacteria and datum/data pose problems. Bacteria is widely assumed to be collective, and bacterium and datum are so seldom used that they often raise doubts. Data is currently both plural (‘The data are available’) and collective (‘How much data do you need?’), and is often therefore a controversial usage issue. Curriculum and memorandum have two plural forms: curricula, curriculums and memoranda, memorandums. The medium/media contrast is complex and extremely controversial. Among spiritualists, the plural of medium is mediums. In linguistics, it is both media and mediums. In the media, it is media, the singular medium often being overlooked, so that media is used as both plural (‘the media are…’) and singular (‘the media is …’), with the occasional vernacular plural form medias; compare French les médias.

In the late 20c, traditional usage has declined as the number of people involved in technical and academic discourse has increased. Contrasting plurals are common: cactus, formula, referendum often have the technical plurals cacti, formulae, referenda and the popular plurals cactuses, formulas, referendums. Such usages as a rock strata, a good criteria, this phenomena is widespread all occur frequently, with the plurals stratas, criterias, phenomenas. They are disliked (often intensely) not only by purists but by many who consider themselves liberal in matters of usage. Purism, however, also has its barbarisms, such as the quasiclassical plurals octopi and syllabi for octopus and syllabus, competing with octopuses and syllabuses. (The Greek plurals for these words are, respectively, octṓpoda and sullabóntes.)