[on-lap, awn-]

noun Geology.

the advance of a sea beyond its former shore.
the layer or layers of sedimentary rock formed on a sea floor as a result of this advance.

Compare offlap.

Origin of onlap

First recorded in 1945–50; on + lap3 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019