Arcana

The noun 'arcana' is a word from Latin. In its own language, its singular and plural forms are regular enough; but in English, they seem irregular. The forms that should be used in academic writing are given here:

The common form	Original language	singular	plural	Remarks
arcana	Latin	arcanum	arcana	'hidden things'; those that were only revealed to initiates. (cf. English adjective arcane)

For further examples of, and articles about, the singular and plural forms of nouns derived from foreign languages, click on one of the categories listed below.

The singular form arcanum does exist in English, but is little used.

You may also want to see -um in Latin.