What is "spacefiller"

n. 1 (context philately English) A stamp in poor condition used to fill a space in an album until a better quality example becomes available, or if the better example is prohibitively expensive. 2 (context cellular automata English) A pattern that grows quadratically by filling space with still life.

It resembles a breeder in that both types of patterns have a quadratic growth rate in their numbers of live cells, and in both having a three-component architecture. However, in a breeder the moving part of the breeder (corresponding to the stretcher) leaves behind a fixed sequence of glider guns which fill space with gliders, moving objects (gliders or spaceships) rather than still life patterns. With a spacefiller, unlike a breeder, every point in the space eventually becomes part of the space-filling still life pattern.