also: ひし
noun
mid-ocean sandbank
Archaic term for a sandbank or shoal in the open sea; not used in modern Japanese outside of historical or literary contexts.
古い海図には「洲」と記された浅瀬がいくつも見られる。
On old nautical charts, many shoals marked as '洲' can be seen.
砂州 is the modern term for a sandbar or sandbank, typically formed along coasts or in rivers, while 洲 is an archaic word specifically for a mid-ocean sandbank.
The historical derivation is uncertain; the kanji 洲 is conventionally associated with sandbanks or islets in water.