(philately) 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.
2008 May 31, Dave Greene, “Life: B37/S23 - A Chaotic Universe.”, in comp.theory.cell-automata[1] (Usenet):
Spacefiller patterns might possibly even be within reach of current search programs; these would have much more predictable (and higher) growth rates from the start.
2009 December 16, Mark Jeronimus, “WireWorld Extendable 3”, in comp.theory.cell-automata[2] (Usenet):
Other things that can be built (I see no reason why not) are a self-replicator, quadratically growing spacefiller, programmable computer/constructor, self-growing prime number sieve.