A tip (also known as gratuity) is a small amount of money received by some service sector professionals from persons they serve, in addition to or instead of a formally required payment. These transactions are governed by social custom.
A popular etymology for tip states it is an acronym that stands for "to insure promptness". However, the Oxford English Dictionary states that it is derived from the English thieves' slang word tip, meaning "to pass from one to another". The notion of a stock tip or racing tip is from the same slang.