'''Human-in-the-loop''' or '''HITL''' is defined as a model that requires human interaction.<ref>"DoD Modeling and Simulation (M&S) Glossary", DoD 5000.59-M, [[United States Department of Defense|DoD]], January 1998 [http://www.dtic.mil/whs/directives/corres/pdf/500059m.pdf]</ref><ref>Karwowski, Waldemar, ''[httphttps://books.google.com/books?id=3hkbHSV81XMC&pg=PA2407 International encyclopedia of ergonomics and human factors]'', ISBN 0-415-30430-X, 9780415304306, [[CRC Press]], 2006</ref> HITL is associated with [[modeling and simulation]] (M&S) in the [[live, virtual, and constructive]] [[Taxonomy (general)|taxonomy]]. HITL models may conform to [[human factors]] requirements as in the case of a [[mockup]]. In this type of simulation a human is always part of the simulation and consequently influences the outcome in such a way that is difficult if not impossible to reproduce exactly. HITL also readily allows for the identification of problems and requirements that may not be easily identified by other means of simulation.
HITL is often referred to as interactive simulation, which is a special kind of physical simulation in which physical simulations include human operators, such as in a [[flight simulator|flight]] or a [[driving simulator]].
