1A 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.
隣の家との境に低い塀がある。
There is a low wall at the boundary with the neighboring house.
2A 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.
3An area, region, or realm; mostly seen in literary language or in compounds (環境, 秘境, etc.); when used alone it is usually read きょう.
この物語の舞台は、現実と夢の境（きょう）にある。
The setting of this story lies in the realm between reality and dreams.
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.
4A psychological or spiritual state — a level or stage of mind, in a somewhat literary usage (read きょう).
長年の修行の末、彼はついに無心の境（きょう）に達した。
After years of training, he finally reached a state of selflessness.
5In 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.