What is Core–mantle boundary; inner-core boundary?

The core–mantle boundary is the seismic discontinuity between the solid mantle and the liquid outer core, at a depth of $2891\,\mathrm{km}$; there the P speed drops from about $13.7\,\mathrm{km}/\mathrm{s}$ to about $8.1\,\mathrm{km}/\mathrm{s}$, and S waves stop. The inner-core boundary, at a depth of $5150\,\mathrm{km}$, separates the liquid outer core from the solid inner core; there the P speed jumps from about $10.4\,\mathrm{km}/\mathrm{s}$ to about $11.0\,\mathrm{km}/\mathrm{s}$.