The going under of something into water — or the state of being fully covered by it. You meet it in careful, technical, or legal writing: reports on floods, drowning, laboratory method, ship accidents. 'Death by submersion' is the phrase a coroner uses where an ordinary person says 'drowning'. It can name a moment ('the submersion of the vessel took four minutes') or a condition ('after long submersion the wood was black and soft'). Everyday English usually prefers other words — 'sinking', 'being under water', 'flooding' — so if a sentence sounds plain, 'submersion' is probably the wrong choice of register. Writers also stretch it to non-water things, meaning being totally covered or swallowed up: 'submersion in a new culture', 'submersion in debt', though 'immersion' is the more usual word for the good, deliberate kind of total involvement. It is normally uncountable.

浸沒、沉入水中（多用於報告、法醫、工程等正式場合）。日常說「淹沒／沉下去」通常不會用這個字。