The term 'La Scala Number' refers to a type of musical composition that originated in the 18th century. It is a combination of an ARIA and a SOLO, both of which are four-letter words. An ARIA is a type of song or vocal piece that is usually accompanied by instruments and is often used in operas. A SOLO is a piece of music performed by one person, typically a singer or instrumentalist. La Scala Number is a type of musical form that combines these two elements.