Seismic discontinuity, Moho, core–mantle and inner-core boundaries কী?

A seismic discontinuity is a surface inside the Earth across which the seismic speeds, the density or both change abruptly, over a thickness small compared with a seismic wavelength. The Mohorovičić discontinuity (or Moho) is the base of the crust, where $v_P$ jumps to about $8\,\mathrm{km}\,\mathrm{s}^{-1}$. The core–mantle boundary is the surface between the solid silicate mantle and the liquid outer core; the inner-core boundary is the surface between the liquid outer core and the solid inner core.