The term 'hollow center' can refer to two different crossword solutions: ELS (3 letters) and DOUBLEL (7 letters). ELS stands for Elevated Light Standard, which is a type of street light that is mounted on a tall pole. DOUBLEL stands for Double-Lined Street Lighting, which is a type of street lighting system that uses two rows of lights to provide illumination. Both of these solutions are used in urban areas to provide better visibility and safety for pedestrians and drivers.