The term 'luau instrument' refers to two instruments commonly used in traditional Hawaiian luau music. The first is the uke, a three-letter word, which is a small four-stringed guitar-like instrument. The second is the ukulele, a seven-letter word, which is a larger four-stringed instrument that is similar to the uke but has a larger body and a longer neck. Both instruments are popular in Hawaiian music and are often used in luau performances.