What is Pyjamas ?
Pyjamas is (plural noun) a light shirt and trousers which you wear in bed I bought two pairs of pyjamas in the sale. When fire broke out in the hotel, the guests ran into the street in their pyjamas. (NOTE: A pair of pyjamas means one shirt and one pair of trousers. The US spelling is pajamas.)
source: Easier English, Student Dictionary Upper Intermediate Level