止境 is a noun meaning 'end' or 'limit', but it is almost always used in negative constructions, especially 无止境 (wú zhǐjìng, 'endless') and 永无止境 (yǒng wú zhǐjìng, 'never-ending'). You won't often hear it in positive sentences like 'this is the end' — instead, it describes something that has no boundary or conclusion, such as learning, exploration, or progress.