The Italian word for the loose-fitting garments we wear to bed is pigiama. In English, we use the plural pyjamas to talk about a single pair of pyjamas, whereas Italian has a singular and plural form (pigiama / pigiami).
Pigiama is sometimes considered invariable, which means that its form does not change in the plural. This means that both i pigiami and i pigiama (the pyjamas) are correct.
Along with pigiama itself, the term pigiama party has also entered the Italian language, referring, as it does in English, to a night-time party where pyjamas or nightgowns are worn.