This is the 4349th most frequent Russian word.
боковой
Lateral; side.
The word 'боковой' here means 'side', describing the position of the entrance.
Боковой вход оказался закрытым.
The side entrance was found to be closed.
The word 'боковые' here refers to 'side', modifying 'mirrors' to indicate their position on the vehicle.
Он оглядывался, проверяя боковые зеркала автомобиля.
He glanced around, checking the side mirrors of the car.
The word 'боковой' here means 'side', describing the position of the panel on the device.