| In words borrowed from Latin, the Latin plural forms are used. |
 |  |
| SINGULAR—US | PLURAL – i |
| cactus | cacti cactuses |
| fungus /ˈfʌŋ-gəs/ | fungi /ˈfʌn-dʒaɪ, ˈfʌŋ-gaɪ/ |
| nucleus /ˈnu-kli-əs/ | nuclei /ˈnu-kli-ˌaɪ/ |
| stimulus | stimuli |
| syllabus /ˈsɪl-ə-bəs/ | syllabi /ˈsɪl-ə-baɪ/ |
| SINGULAR—ON, —UM | PLURAL –A |
| phenomenon Greek / Latin | phenomena /fɪˈ-nɒ-mə-nə/ |
| criterion Greek / Latin | criteria |
| bacterium Latin | bacteria |
| curriculum Latin | curricula |
| datum Latin | data /ˈde-ɪtə, ˈdæ-tə, ˈdɑ-tə/ |
| medium Latin | media |
| memorandum Latin | memoranda |
| SINGULAR—A | Plural –AE |
| formula /ˈfɔrmyələ/ Latin | formulas / formulae /ˈfɔrm-yə-li/ |
| vertebra Latin | vertebrae |
| SINGULAR—IX, —EX | Plural –ICES, –ES, |
| appendix /əˈpɛn-dɪks/ Latin | appendices / ixes /əˈpɛn-də-siz/ |
index Latin
| indices / indexes |
