1. Also called plain lap. a joint, as between two pieces of metal or timber, in which the pieces overlap without any change in form.
2. any of various joints between two members, as timbers, in which an end or section of one is partly cut away to be overlapped by an end or section of the other, often so that flush surfaces result.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.