What term describes a sum of money given to a server for their service, also known as a tip?

The term that describes a sum of money given to a server for their service is "gratuity." Gratuity is a customary payment made by customers to express their appreciation for the service provided, typically in the context of dining or hospitality. It reflects the customer's satisfaction and is often an important part of the income for service staff in the restaurant and food service industry.

The other terms listed do not relate to this concept. An ESP system typically refers to an electronic sales platform or a specific technology used within the hospitality industry, while a franchise agreement is a legal contract that establishes the rights and obligations of parties in a franchising relationship. A food service company refers more broadly to businesses involved in food provision and management rather than the act of tipping itself.