means An imitation of a real process or system, run so you can study, train, or predict without the risks or costs of the real thing.

from From Latin 'simulare,' to make like or imitate, rooted in 'similis,' meaning 'like' or 'similar' — the same family that gives us 'similar,' 'assemble,' and the slippery 'dissimulate.' Early English use leaned toward the sense of feigning or pretending; the modern technical meaning, a working model that mimics reality, is a later development that grew up alongside computing and engineering.