There are many synonyms for the phrase "overnight party." This type of social gathering can be known as a sleepover, slumber party, or even a pajama party. Additionally, it may be referred to as a lock-in, where guests are not allowed to leave until the following day. Another term used is an all-nighter, which suggests staying up late into the night. Some may use the phrasing "evening-long party" if the guests are only present until late at night. Regardless of the name, an overnight party is a fun and unique way to spend time with friends and celebrate a special occasion.