The word "pajamas" in U.S. English is sometimes spelled "pyjamas" in British English. Sometimes they're nicknamed nicknamed "PJs," "jim-jams" or "jammies."
The word comes from a Persian word that means "leg garment."
There are many different types of pajamas. Some have short sleeves, som have long sleeves. Some are one piece, some are two pieces.
Some people wear their pajamas outdoors. Here's an old photo (taken between 1930-1940) of a woman posing at the seaside in her "beach pajamas":
In some parts of Asia, it's commonplace for adults to wear their pajamas outside in their local neighborhoods:
And in 2012, there was a celebrity trend in the U.S. of public pajama fashion.
Not everyone's on board with wearing pajamas outside of the bedroom, though.
In 2011, a Vermont high school prohibited pajamas and slippers because it was felt they were a safety hazard.
In 2012, a Louisiana Commissioner proposed an ordinance prohibiting people from wearing pajamas in public.
What's most important in pajamas: that they're comfy!
A Brief History Of Pajamas - from Sit'n'Sleep
History Of Pyjamas - Lifestyle Lounge
History Of Pajamas - Deshabille