diner

someone who is eating a meal, especially in a restaurant

in the US, a small informal and inexpensive restaurant, often at the side of the road

someone who eats a meal, esp. in a restaurant:

Examples:

A diner is a cheap, informal restaurant that traditionally is long and narrow and looks like part of a train, in which people sit at a counter (= long table) and also in booths (= partly enclosed areas on either side of a table).