A threshold is the strip of wood or stone at the bottom of a doorway that you step over when entering a room or building. You've crossed thousands of thresholds in your life, probably without noticing most of them. Some old houses have raised thresholds that stick up noticeably, while modern buildings often have flat ones nearly flush with the floor.

The word also means the point where something begins or changes. A pain threshold is the level where discomfort starts to really hurt. Scientists talk about a hearing threshold, the softest sound a person can detect. In a classroom, you might hear about a “threshold for passing,” meaning the minimum score needed to succeed.

When you stand on a threshold between rooms, you're literally between two spaces. That physical meaning helps explain the word's broader use: a threshold is always that critical point where one state ends and another begins. A swimmer might be on the threshold of breaking a school record, meaning they're right at the edge of achieving something significant.