1. n. [Geology]

The termination of shallowly dipping, younger strata against more steeply dipping, older strata, or the termination of low-angle reflections in seismic data against steeper reflections. Onlap is a particular pattern of reflections in seismic data that, according to principles of sequence stratigraphy, occurs during periods of transgression.

See: apparent dip,  downlap,  pinch-out,  retrogradation,  sequence stratigraphy,  transgression