[on-lap, awn-]
noun Geology.
  1. the advance of a sea beyond its former shore.
  2. 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. 2018