diciottesimo

Diciottesimo is the Italian ordinal form corresponding to the number eighteen. It is used to indicate position in a sequence, such as the eighteenth item in a list or the eighteenth day of a month. In addition to its ordinal function, diciottesimo can also appear as a noun meaning the eighteenth part of a whole.

The word is formed from diciotto, meaning eighteen, plus the suffix -esimo. It has gendered forms to agree with the noun it modifies: diciottesimo (masculine singular), diciottesima (feminine singular), diciottesimi (masculine or mixed plural), and diciottesime (feminine plural).

Common uses include dating and dating-related references, such as diciottesimo giorno, diciottesima pagina, or diciottesima edizione. It is also used for centuries, especially in older or more formal styles, where diciottesimo secolo or XVIII secolo is encountered, though the numeral form XVIII is often preferred in scholarly writing.

As a noun, diciottesimo denotes one part out of eighteen. For example, un diciottesimo di litro means one eighteenth of a liter. Plural usage is i diciottesimi, indicating multiple eighteenth parts or instances. The term is interchangeable with the symbolic representation XVIII when used in contexts like numbering, fractions, or dating, depending on formality and style.

Related terms include ordinal forms for other numbers (ventesimo, ventunesimo) and fractional nouns in Italian, which follow similar gender and plurality rules.