A pajama party, also called a slumber party, is an overnight party that pre-teen or teenaged girls participate in. The party guests bring their pajamas or sleepwear and prepare to stay up most of the night playing silly games, watching scary movies, eating snacks, giving each other makeovers, things like that. Usually everyone brings a sleeping bag and the whole group camps out on the living room floor together. Often there are "rules" such as the first person who falls asleep will have her underwear put in the freezer by the others (so when she wakes up in the morning it will be frozen). It's mainly a way to get together and giggle and be silly and stay up much later than you usually do.