nesting

noun3

1. •
   The process by which a bird nests.
2. •
   An arrangement by which one thing is nested inside another.

   • the nesting of Russian dolls
3. •
   The enclosure of one self-contained element of code inside another.

   • The positioning of a control structure inside another control structure is called nesting e.g. nested-IF and nested-loop. — 2013