Definitions

1. noun

a part of something that overlaps or extends below something else

“The underlap of the rug was uneven and had to be adjusted.”

2. verb

to extend or overlap something so that it is partially below something else

“The new roof will underlap the existing one to ensure waterproofing.”

3. verb

to be or become partially below or under something

“The new wall underlaps the old one by several inches.”