'Waiter' is the most common word in British and international English for a person who serves food in a restaurant. 'Server' is more common in North America and is fully gender-neutral, making it the preferred term in many modern workplaces. 'Waitress' is the female-specific counterpart; its use has declined as gender-neutral terms become standard. 'Waitstaff' is a collective noun — you would say 'the waitstaff are friendly' but not 'a waitstaff' for an individual. 'Steward' is used in more formal contexts such as cruise ships, first-class train carriages, or private clubs, and implies a wider range of service duties beyond food service alone.
Formed from the verb 'wait' (meaning 'to attend to, to serve' — a sense dating from the 14th century) plus the agent suffix '-er' ('one who does'). First recorded in the 15th century referring to an attendant or servant in a general sense. The specific meaning of 'person who serves food in a restaurant' emerged in the late 18th century, as public dining establishments became common in European cities.