什么是 Decay constant？

For each unstable nuclide there is a constant $\lambda$, its decay constant (unit $\mathrm{s}^{-1}$), such that during any short interval $\dd t$ each nucleus has probability $p = \lambda\,\dd t$ of decaying — the same for every nucleus of the species, whatever its age, and untouched by temperature, pressure or chemistry.