cavity; hollow; cave; hollow
cave; cavern; den; grotto
insight; discernment; perception; perspicacity; acumen
sinus; cavity; antrum; neighborhood (administrative division in North Korea)
limestone cave; limestone cavern
residence of a retired emperor; dwelling of a former emperor; retired emperor; former emperor; emperor emeritus
making hollow; making meaningless; deindustrialization; deindustrialisation
tree hollow; hollow in a tree
service tunnel (cables, gas, etc.)
snow cave (in mountaineering); snow hole
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
cavity resonator; microwave resonance cavity; rhumbatron
hollow brick (used for thermal isolation or moisture proofing)
sinus node; sinoatrial node
cerebral venous sinus thrombosis; CVST
(animal) whiskers; vibrissae
druse; globular calcium oxalate crystals, often lining a rock cavity
dental cavity (due to caries)
sand martin (Riparia riparia)
brown-throated martin (Riparia paludicola); brown-throated sand martin