宿 (sù) is a bound form (b.f.) used in Chinese astronomy to refer to one of the 28 lunar mansions (二十八宿) — the traditional Chinese equivalent of constellations. You never use 宿 alone as a standalone word; it always appears in compounds like 星宿 (constellation) or 二十八宿 (the 28 mansions). The term is distinct from the Western concept of 'constellation' because the 宿 system divides the sky along the moon's path and has cultural significance in Chinese astrology and calendar.