Is this nightwear or underwear?

[diamondlizard]

here

Could work for either I think.

Underwear, but could be worn as nightwear I guess - I'd wear the top part definitely, but prefer something looser and longer on the bottom.

Underwear for me,
DD1 would agree with MrsD and wear the top to bed. (Very hot up in her bunk, PJ tops are often replaced by vests like that).