a person eating a meal (especially in a restaurant)
"The diner sat quietly at the corner table, enjoying his lunch alone."
a passenger car where food is served in transit
"The weary traveler stopped at a roadside diner to grab a hot meal while continuing their journey across the state."
a restaurant that resembles a dining car
"The old diner on Route 65 features stainless steel counters and red vinyl booths, perfectly resembling a classic train dining car."
One who dines.
"The diner enjoyed his meal at the small table by the window."
The word diner is a straightforward combination of the verb to dine and the suffix -er. It was first used to describe someone who eats dinner or an establishment that serves evening meals before coming to mean any casual restaurant open late into the night.