Sleepover

A sleepover is a social occasion whereby a friend stays overnight at another friend's house. A pajama party or slumber party is essentially the same thing, but may involve multiple people. A lock-in again involves multiple people, but it differs in that it is held in a setting other than a private home, such as a school or church. In all of the aforementioned, it is implicit that the participants are young people, perhaps children or teenagers. The goal of these events is simply to have fun with others. Unlike a traditional play date, however, sleepovers (and its variants) up the ante by having the participants sleep together. This may introduce some social dynamics not normally present in a typical get-together.
Characteristics.
Sleepovers, by their definition, entail long periods of time without adult presence, in an enclosed space, while in the presence of other children. In a lock-in, the participant is also likely far from home (and, true to the term, locked in whatever building they are in). Therefore, a certain level of independence is necessary. In fact, the sleepover is often seen as a rite of passage for a young child or teenager, as they begin to assert independence and to develop social connections outside the immediate family.
Teen sleepovers.
Beginning in the 1990s, commentators wrote about a perceived new trend of parents endorsing sleepovers for teenagers, with both boys and girls staying overnight together. While some writers decried the trend, others defended it as a safer alternative to teenage dating outside the house.