The plural of apparatus is apparatus or apparatuses. A set instruments, equipment or materials designed with a specific purpose in mind. Apparati or Apparatuses? The word apparatus has a Latin root but unlike many nouns in Latin its plural is not formed by replacing the -us ending with an -i.

The plural form of the word “apparatus” is “apparatuses”, according to the Cambridge Dictionary. The term originally comes from Latin, which uses “apparatus” for both singular and plural; however, as English has adapted, the form “apparatuses” has become the standard plural version.