The strip of wood, stone, or metal at the bottom of a doorway that you step over to enter a room or building. It also means the point at which something starts to change or have an effect.
From Old English 'þrescwald' or 'þerxold,' probably meaning 'place where you thresh (trample) grain.' The exact origin is debated, but it seems tied to stepping and trampling at a doorway. Over time, it shifted from a physical doorstep to any starting point or limit.
Scientists talk about 'pain thresholds' and 'hearing thresholds' as the point where your senses finally notice something. That’s just a mental doorway: below it, nothing; above it, experience. The house entrance you cross every day quietly became a metaphor for every kind of beginning.
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