洞|ほらhora[2]odaka (tail-high)

1. nounhollow; cavity; hole; cave

• Synonym穴あなNeutralThe broadest word for holes, pits and openings, as in 「穴を掘る」; 「洞（ほら）」 is narrower, for hollows in cliffs or tree trunks.
• Synonym空洞くうどうWrittenA written/formal term for a hollow cavity, also used figuratively for emptiness as in 「空洞化」; 「洞」 is a concrete, physical hollow.
• Synonym洞穴ほらあなNeutralA cave in a cliff or rock; works in both speech and writing. 「洞」 is seldom used alone and leans toward hollows in cliffs and trunks.
• Synonym洞窟どうくつNeutralA deep cave in a cliff or rock; common in daily use as well. 「洞」 is rarely used alone and covers hollows generally, including tree-trunk hollows.
• Synonym岩屋いわやNeutralA natural rock cave or a dwelling carved into rock; 「洞」 focuses on the hollow space itself.
• Related穴ぼこあなぼこCasualA colloquial word for a 「穴」 or 「窪み」: holes and dents in roads, walls and the like; 「洞」 is for hollows in cliffs or tree trunks.
• Related虚うろNeutralThe hollow inside a tree trunk or similar, as in 「木のうろ」; 「洞（ほら）」 leans toward caves in cliffs and rock, though it can also mean a tree hollow.
• Related穴隙けつげきWrittenA written/formal term for a narrow crevice in rock or a wall; 「洞」 is for larger hollows.
• Related岩窟がんくつWrittenA written/formal word for a rocky cave, as in 「岩窟王」; 「洞」 centers on the hollow itself.
• Related岩穴いわあなNeutralA cave in rock; 「洞」 also covers hollows inside tree trunks.
• Related岩室いわむろWrittenA written, archaic term for a room-like rock cave; 「洞」 leans toward hollow cavities in general.

Use 「洞（ほら）」 for a hollow cavity in a cliff or tree trunk; when read 「どう」 it mostly appears in compounds like 「洞窟」. For ordinary holes pick 「穴」, for caves 「洞穴」 or 「洞窟」, and for a hollow inside a tree 「虚（うろ）」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 崖がけや大木たいぼくの幹みきなどにできた中空なかぞらの穴あな。