Pj's and Underwear

Pj's noun - (usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers.

Underwear and pj's are semantically related. In some cases you can use "Underwear" instead a noun "Pj's".

Underwear noun - Clothing intended to be worn underneath other clothing.

Pj's and underwear are semantically related. Sometimes you can use "Pj's" instead a noun "Underwear".