A word that is impossible to define without using the word itself, for when you don’t want an empty hole

noun

1. Material that fills in something (such as a hole or the spaces between a building's structural members)

   "volcanic infill"

2. New buildings constructed in the space available between existing structures

   "Urban infill, a big theme these days in city planning, is essentially the opposite of "urban sprawl." As cities grow, planners are looking for ways to pack more people into places that are already developed."