A "missing sock" refers to a common household phenomenon where one sock from a pair cannot be found after laundry. This often occurs due to various reasons, such as socks getting stuck in the washing machine or dryer, or being misplaced during sorting. The mystery of the missing sock has led to various theories and jokes, with some people humorously attributing it to a mythical creature known as the sock monster. Many individuals have developed strategies to prevent this issue, such as using a mesh laundry bag or safety pins to keep pairs together during washing.