A nap is a short period of time spent sleeping. Naps are typically held at daytime, such as for about one hour after lunch, in the early afternoon.
Small children are often put down for a nap every day, but also many adults like to hold a nap (particularly elderly people). In many countries, naps form an essential part of the culture, such as the Spanish and Latin American siesta.
Note that the term nappy refers not to a nap but to a diaper.
Naps and spanking
Naps are popular in ageplay (especially ABDL roleplay) and in spanking stories that involve minors, such as Melody's Stories. While a nap is not really a punishment, being put in pajamas (or undressed to the underwear) at bright day and laid to bed for a nap ("lights off, no getting up") can make a person feel treated like a small child, and also emphasizes the guardian's authority.
Naps as a punishment
Being put down a nap can be used as a short term isolation punishment. Where the misbehavior is caused by cranky over-tiredness this is more a correction than a punishment. However when the child isn't actually tired then it function like a confined to bed variation of a time-out with small child overtones that an add an element of humiliation for those that believe they have outgrown the need for naps.