IPA: /ˈɪn.fɪl/
KK: /ˈɪnˌfɪl/
A material or substance used to fill empty spaces in a structure or area, often for construction or development purposes.
The architect decided to use brick as the infill for the building's walls.
Infill is formed from "in-" (meaning into) and "fill" (from Old English "fyllan", meaning to make full). The word describes the act of filling something in or making it full from the inside.
Think of putting something 'into' a space to 'fill' it up — that's what infill means.