noun
passenger seat; front passenger seat
Refers specifically to the seat next to the driver in a car or other vehicle. The term is neutral and widely used in everyday conversation.
彼はいつも助手席に座る。
He always sits in the passenger seat.
助手席のシートベルトを締めてください。
Please fasten the passenger seat belt.
後部座席 refers to the back seats of a vehicle, in contrast to the front passenger seat (助手席).
Compound of 助手 (assistant) and 席 (seat), literally 'assistant's seat'. The term likely originated from the idea that the person sitting there assists the driver, e.g., by navigating or handling controls.