From IFWiki

A simulationist work is any that uses a consistent and detailed world model to produce results, rather than having only special cases hard-coded by the author. In this sense, a work of IF may be said to be more or less simulationist depending on how much it depends on the programmed world model; moreover, different aspects of the same game may be simulated to different degrees. A game might have a detailed simulation of the behavior of liquids, for instance, while relying entirely on special-case coding to handle the behavior of NPCs.