什么是 Terminal speed？

Example 26.16 (Terminal orders of magnitude)

A flat-out skydiver: about $55\,\mathrm{m}/\mathrm{s}$ ($200\,\mathrm{km}/\mathrm{h}$). A raindrop: about $9\,\mathrm{m}/\mathrm{s}$ — in vacuum, $2000\,\mathrm{m}$ of fall would deliver it at $\sqrt{2gh} \approx 2.0 \times 10^{2}\,\mathrm{m}/\mathrm{s}$. A feather: about $1\,\mathrm{m}/\mathrm{s}$, reached within centimeters — the secret of its slowness: it spends its whole fall at terminal speed; the hammer never gets near its own.