Wikipedia
MilSim is an abbreviation of military simulation, and refers to military simulations conducted by civilians for entertainment purposes. There are several forms of MilSim: airsoft games, paintball games, and video games simulating military scenarios and tactics.
Weapons used in MilSim are commonly airsoft guns. Airsoft guns are used more often in MilSim than paintball guns due to their availability as replica weapons without any modification.
In the past, due to the unrealistic nature of the performance of airsoft and paintball weapons, they were seldom used in real-world training environments. Instead, specialized non-lethal ammunition or blanks are used in conjunction with infrared targeting systems. More recently however, advances in airsoft replica authenticity and function have led numerous law enforcement and military units using them to train in CQB (Close Quarters Battle) and CQC (Close Quarters Combat) environments. Airsoft and Paintball guns though previously not considered realistic by the US Department of Defense or major police departments for training purposes are becoming more so due to increases in realism and function, especially airsoft.
MilSim most often falls into two different categories: reenactment and simulation. Reenactments focus on a particular event in history and aim to provide the most accurate representation of the time period, events, and outcomes of the historical event. Simulation, however, may only focus on a particular time period and representation of which course a particular situation may have taken in history.