A costume party is an event where people dress up in costumes and often take part in activities such as dancing, playing games, or watching performances. The two crossword solutions for the term 'costume party' are BALL (4 letters) and MASQUERADE (10 letters). A BALL is a formal event where people dress up in their finest attire and dance to music. A MASQUERADE is a type of costume party where guests wear masks and costumes to conceal their identity.