A border or boundary between places or areas (usually read さかい).
川が町と村の境になっている。
The river forms the boundary between the town and the village.
市の境を越えて隣の町に行った。
I crossed the city boundary and went to the neighboring town.
国境（こっきょう）を中心とした会議が開かれた。
A conference focusing on national borders was held.
A turning point or watershed after which things change markedly (often used as '〜を境に').
入社を境に生活が一変した。
My life changed completely after joining the company.
その事件を境に彼の考え方が変わった。
His way of thinking changed after that incident.
An area, region, spot, or environment (this usage often appears in written or compound words and is frequently read きょう).
境界線を引いて、危険区域の境を示した。
They drew a boundary line to mark the limits of the danger area.
自然と都市の境（きょう）で生態系が変化する。
The ecosystem changes at the boundary between nature and the city.
A psychological or mental state; a borderline mental condition or state of mind (sometimes read きょう).
彼は喜びと悲しみの境に立っているような表情だった。
He had an expression as if standing on the border between joy and sorrow.
瞑想を続けて、新しい境地（きょうち）に達したと感じた。
After continuing meditation, I felt I had reached a new state of mind.
In Buddhism, an object of cognition — something perceptible by the sense organs or the mind (a technical/religious term).
仏教では六境（ろっきょう）という概念があり、感覚対象の分類に用いられる。
In Buddhism there is the concept of the six 'kyo' (rokkyō), which is used to classify objects of sense perception.