Definition: image or representation
This seems to be used to refer to an actual physical thing that simulates something else. I think we use the word “simulation” for this quite a bit, though that word seems to be tilted to the experience of a simulacrum.
So, a photograph, for instance, might be a simulacrum (noun) that simulates (verb) something and therefore provides a simulation (noun) to its observers.
Why I Looked It Up
An episode of Buck Rogers was centered around an alien being who created an exact copy of Searcher and its inhabitants. The fake humans were referred to as “simulacrums” several times.