Pajamas (American English) or pyjamas (Commonwealth English), sometimes colloquially shortened to PJs, jammies, jim-jams, or in South Asia, night suits, are several related types of clothing worn as nightwear or while lounging. Pajamas are soft garments derived from the Indian and Iranian bottom-wear, the pyjamas, which were adopted in the Western world as nightwear.

According to the Oxford English Dictionary, the word pyjama is a borrowing via Urdu from Persian. Its etymology is:

Urdu pāy-jāma, pā-jāma and its etymon Persian pāy-jāma, pā-jāma, singular noun < Persian pāy, pā foot, leg + jāma clothing, garment (see jama n.1) + English -s, plural ending, after drawers.

The worldwide use of pyjamas (the word and the clothing) outside the Indian subcontinent is the result of adoption by British colonists and soldiers in the Indian subcontinent in the 18th and 19th centuries, and the British influence on the wider Western world during the Victorian era. Pyjamas had been introduced to England as "lounging attire" as early as the seventeenth century, then known as mogul's breeches (Beaumont and Fletcher) but they soon fell out of fashion. The word pyjama (as pai jamahs, Paee-jams and variants) is recorded in English use in the first half of the nineteenth century. They did not become a fashion in Britain and the Western world as sleeping attire for men until the Victorian period, from about 1870.

Hobson-Jobson: A Glossary of Colloquial Anglo-Indian Words and Phrases (1886) summarizes the state of usage at the time (s.v. "pyjammas"):

Such a garment is used by various persons in India e.g. by women of various classes, by Sikh men, and most by Mohammedans of both sexes. It was adopted from the Mohammedans by Europeans as an article of dishabille [highly casual clothing] and of night attire, and is synonymous with Long Drawers, Shulwaurs, and Mogul-Breeches [...] It is probable that we English took the habit like a good many others from the Portuguese. Thus Pyrard (c. 1521) says, in speaking of Goa Hospital: "Ils ont force caleçon sans quoy ne couchent iamais les Portugais des Indes" [fr., "They have plenty of the undergarments without which the Portuguese in India never sleep"] [...] The word is now used in London shops. A friend furnishes the following reminiscence: "The late Mr. B—, tailor in Jermyn Street, some on 12 years ago, in reply to a question why pyjammas had feet sewn on to them (as was sometimes the case with those furnished by London outfitters) answered: "I believe, Sir, it is because of the White Ants."

Traditional pajamas consist of a shirt-and-trousers combination made of soft fabric, such as flannel or lightweight silk. The shirt element usually has a placket front and sleeves with no cuffs.

Pajamas are usually worn as nightwear with bare feet, and can be worn with or without undergarments. They are also worn for comfort by people in their homes.