cavity; hollow; cave; hollow
cave; cavern; den; grotto
residence of a retired emperor; dwelling of a former emperor; retired emperor; former emperor; emperor emeritus
service tunnel (cables, gas, etc.)
venous sinus; sinus venosus
dongxiao (Chinese bamboo flute similar to a shakuhachi)
sick sinus syndrome; sinus node dysfunction; SSS
sinoatrial; (arch.) inner bedroom; (arch.) brothel
sinus arrhythmia; irregular heartbeat
sinus node; sinoatrial node
(animal) whiskers; vibrissae
sand martin (Riparia riparia)