What is a small gift or sum of money tendered for a service provided called?

The term that best describes a small gift or sum of money given for a service rendered is 'gratuity.' This is commonly understood as a voluntary payment given in recognition of the service you have received, often seen in contexts like dining, where patrons leave a tip for good service. Gratuities serve as a token of appreciation for the effort put in by the service provider, beyond the base price of the service.

In contrast, a donation typically refers to a larger, often charitable contribution made without the expectation of receiving something in return, while an offering is usually associated with religious or ceremonial contexts, given as a sign of devotion or reverence. Honorariums are generally paid to professionals for their service in speaking or providing a lecture, and while they can be seen as similar to a gratuity, they often carry a more formal connotation and are not necessarily a small, discretionary amount aimed at tipping for everyday services. Thus, the context of a gratuity makes it the most fitting choice in this case.