Convective overshoot is a phenomenon of convection carrying material beyond an unstable region of the atmosphere into a stratified, stable region. Overshoot is caused by the momentum of the convecting material, which carries the material beyond the unstable region.
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Deep, moist convection in Earth's atmosphere
One example is thermal columns extending above the top of the equilibrium level (EL) in thunderstorms: unstable air rising from (or near) the surface normally stops rising at the EL (near the tropopause) and spreads out as an anvil cloud; but in the event of a strong updraft, unstable air is carried past the EL as an overshooting top or dome. A parcel of air will stop ascending at the maximum parcel level (MPL). This overshoot is responsible for most of the turbulence experienced in the cruise phase of commercial air flights.
Stellar convection
Another example of convective overshoot is at the base of the convection zone in the solar interior. The heat of the Sun's thermonuclear fusion is carried outward by radiation in the deep interior radiation zone and by convective circulation in the outer convection zone, but cool sinking material from the surface penetrates farther into the radiative zone than theory would suggest. This affects the heat transfer rate and the temperature of the solar interior which can be indirectly measured by helioseismology. The layer between the Sun's convective and radiative zone is called the tachocline.
Overshooting in the convective core of more massive stars is critical to the modelling of evolution beyond the main sequence. Overshooting causes the core mass at the end of the main sequence to be larger than would otherwise be expected. This leads to big differences in behaviour on the subgiant and giant branches for intermediate mass stars, and to radical changes in the evolution of massive supergiant stars.