downlap

onlap

toplap

terminations

Lateral termination of strata at their depositional pinchout. lapout may occur at the upper boundary of a sequence, where it is called toplap, or at the lower boundary, where it is called onlap or downlap.

References
Mitchum Jr., R.M., 1977. Seismic stratigraphy and global changes of sea level. Part 11: glossary of terms used in seismic stratigraphy. In: Payton, C.E. (Ed.), Seismic stratigraphy––Applications to Hydrocarbon Exploration, vol. 26. A.A.P.G. Memoir, pp. 205–212.