Pajamas is the usual American spelling of the loose top and trousers people wear in bed. British English writes pyjamas. Same clothes, same word — only the letters change. Americans also use pajama in front of another noun: pajama bottoms, a pajama party. For the whole set, people still say a pair of pajamas, the way they say a pair of trousers. It is home language, not formal. The word came into English from Hindustani (Hindi pājāmā / Urdu pājāma), and before that from Classical Persian pājāma, “trousers”: pā “leg” + jāma “garment”. So it first meant loose trousers; English later used it for night clothes as a set. Though actually, related words in some South Asian languages still mean daytime trousers, not sleepwear. English moved the meaning.