The term virtual reality dates back to German philosopher Immanuel Kant , although its use did not involve technology. Its modern use was popularized by Jaron Lanier in the 1980s. Although it is already quite encompassing, several competing terms related to VR are in common use at present. The term virtual environments predates widespread usage of VR and is presently preferred by most university researchers . It is typically considered to be synonymous with VR; however, we emphasize in this book that the perceived environment could be a captured ``real'' world just as well as a completely synthetic world. Thus, the perceived environment need not seem ``virtual''. Augmented reality (AR) refers to systems in which most of the visual stimuli are propagated directly through glass or cameras to the eyes, and some additional structures appear to be superimposed onto the user's world. The term mixed reality (MR) is sometimes used to refer to an entire spectrum that encompasses VR, AR, and normal reality. More recently, the term VR/AR/MR has even been used to refer to all forms. Telepresence refers to systems that enable users to feel like they are somewhere else in the real world; if they are able to control anything, such as a flying drone, then teleoperation is an appropriate term. For our purposes, virtual environments, AR, mixed reality, telepresence, and teleoperation will all be considered as perfect examples of VR. The most important idea of VR is that the user's perception of reality has been altered through engineering, rather than whether the environment they believe they are in seems more ``real'' or ``virtual''. Thus, perception engineering could be a reasonable term as well.
Unfortunately, name virtual reality itself seems to be self contradictory, which is a philosophical problem which was rectified in  by proposing the alternative term virtuality. While acknowledging all of these issues, we will nevertheless continue onward with term virtual reality. The following distinction, however, will become important: The real world refers to the physical world that contains the user at the time of the experience, and the virtual world refers to the perceived world as part of the targeted VR experience.
Steven M LaValle 2016-12-31