Immersion into virtual reality is a perception of being physically present in a non-physical world. The perception is created by surrounding the user of the VR system in images, sound or other stimuli that provide an engrossing total environment.
The name is a metaphoric use of the experience of submersion applied to representation, fiction or simulation. Immersion can also be defined as the state of consciousness where a "visitor" (Maurice Benayoun) or "immersant" (Char Davies)'s awareness of physical self is transformed by being surrounded in an artificial environment; used for describing partial or complete suspension of disbelief, enabling action or reaction to stimulations encountered in a virtual or artistic environment. The degree to which the virtual or artistic environment faithfully reproduces reality determines the degree of suspension of disbelief. The greater the suspension of disbelief, the greater the degree of presence achieved.
Staffan Björk and Jussi Holopainen, in Patterns In Game Design, divide immersion into similar categories, but call them sensory-motoric immersion, cognitive immersion and emotional immersion, respectively. In addition to these, they add a new category:
Presence, a term derived from the shortening of the original "telepresence", is a phenomenon enabling people to interact with and feel connected to the world outside their physical bodies via technology. It is defined as a person's subjective sensation of being there in a scene depicted by a medium, usually virtual in nature (Barfield et al., 1995).[full ] Most designers focus on the technology used to create a high-fidelity virtual environment; however, the human factors involved in achieving a state of presence must be taken into account as well. It is the subjective perception, although generated by and/or filtered through human-made technology, that ultimately determines the successful attainment of presence. 
Virtual reality glasses can produce a visceral feeling of being in a simulated world, a form of spatial immersion called Presence. According to Oculus VR, the technology requirements to achieve this visceral reaction are low-latency and precise tracking of movements.
Immersive virtual reality is a hypothetical future technology that exists today as virtual reality art projects, for the most part. It consists of immersion in an artificial environment where the user feels just as immersed as they usually feel in consensus reality.
The most considered method would be to induce the sensations that made up the virtual reality in the nervous system directly. In functionalism/conventional biology we interact with consensus reality through the nervous system. Thus we receive all input from all the senses as nerve impulses. It gives your neurons a feeling of heightened sensation. It would involve the user receiving inputs as artificially stimulated nerve impulses, the system would receive the CNS outputs (natural nerve impulses) and process them allowing the user to interact with the virtual reality. Natural impulses between the body and central nervous system would need to be prevented. This could be done by blocking out natural impulses using nanorobots which attach themselves to the brain wiring, whilst receiving the digital impulses of which describe the virtual world, which could then be sent into the wiring of the brain. A feedback system between the user and the computer which stores the information would also be needed. Considering how much information would be required for such a system, it is likely that it would be based on hypothetical forms of computer technology.
A comprehensive understanding of which nerve impulses correspond to which sensations, and which motor impulses correspond to which muscle contractions will be required. This will allow the correct sensations in the user, and actions in the virtual reality to occur. The Blue Brain Project is the current, most promising research with the idea of understanding how the brain works by building very large scale computer models.
The nervous system would obviously need to be manipulated. Whilst non-invasive devices using radiation have been postulated, invasive cybernetic implants are likely to become available sooner and be more accurate. Manipulation could occur at any stage of the nervous system - the spinal cord is likely to be simplest; as all nerves pass through here, this could be the only site of manipulation. Molecular Nanotechnology is likely to provide the degree of precision required and could allow the implant to be built inside the body rather than be inserted by an operation.
A very powerful computer would be necessary for processing virtual reality complex enough to be nearly indistinguishable from consensus reality and interacting with central nervous system fast enough.
Immersive digital environments could be thought of as synonymous with virtual reality, but without the implication that actual "reality" is being simulated. An immersive digital environment could be a model of reality, but it could also be a complete fantasy user interface or abstraction, as long as the user of the environment is immersed within it. The definition of immersion is wide and variable, but here it is assumed to mean simply that the user feels like they are part of the simulated "universe". The success with which an immersive digital environment can actually immerse the user is dependent on many factors such as believable 3D computer graphics, surround sound, interactive user-input and other factors such as simplicity, functionality and potential for enjoyment. New technologies are currently under development which claim to bring realistic environmental effects to the players' environment - effects like wind, seat vibration and ambient lighting.
To create a sense of full immersion, the 5 senses (sight, sound, touch, smell, taste) must perceive the digital environment to be physically real. Immersive technology can perceptually fool the senses through:
Once the senses reach a sufficient belief that the digital environment is real (it is interaction and involvement which can never be real), the user must then be able to interact with the environment in a natural, intuitive manner. Various immersive technologies such as gestural controls, motion tracking, and computer vision respond to the user's actions and movements. Brain control interfaces (BCI) respond to the user's brainwave activity.
Training and rehearsal simulations run the gamut from part task procedural training (often buttonology, for example: which button do you push to deploy a refueling boom) through situational simulation (such as crisis response or convoy driver training) to full motion simulations which train pilots or soldiers and law enforcement in scenarios that are too dangerous to train in actual equipment using live ordinance.
Computer games from simple arcade to massively multiplayer online game and training programs such as flight and driving simulators. Entertainment environments such as motion simulators that immerse the riders/players in a virtual digital environment enhanced by motion, visual and aural cues. Reality simulators, such as one of the Virunga Mountains in Rwanda that takes you on a trip through the jungle to meet a tribe of mountain gorillas. Or training versions such as one which simulates taking a ride through human arteries and the heart to witness the buildup of plaque and thus learn about cholesterol and health.
In parallel with scientist, artists like Knowbotic Research, Donna Cox, Rebecca Allen, Robbie Cooper, Maurice Benayoun, Char Davies, and Jeffrey Shaw use the potential of immersive virtual reality to create physiologic or symbolic experiences and situations.
Other examples of immersion technology include physical environment / immersive space with surrounding digital projections and sound such as the CAVE, and the use of virtual reality headsets for viewing movies, with head-tracking and computer control of the image presented, so that the viewer appears to be inside the scene. The next generation is VIRTSIM, which achieves total immersion through motion capture and wireless head mounted displays for teams of up to thirteen immersants enabling natural movement through space and interaction in both the virtual and physical space simultaneously.
New fields of studies linked to the immersive virtual reality emerges every day. Researchers see a great potential in virtual reality tests serving as complementary interview methods in psychiatric care. Immersive virtual reality have in studies also been used as an educational tool in which the visualization of psychotic states have been used to get increased understanding of patients with similar symptoms. New treatment methods are available for schizophrenia and other newly developed research areas where immersive virtual reality is expected to achieve melioration is in education of surgical procedures, rehabilitation program from injuries and surgeries and reduction of phantom limb pain.
Simulation sickness, or simulator sickness, is a condition where a person exhibits symptoms similar to motion sickness caused by playing computer/simulation/video games (Oculus Rift is working to solve simulator sickness).
Motion sickness due to virtual reality is very similar to simulation sickness and motion sickness due to films. In virtual reality, however, the effect is made more acute as all external reference points are blocked from vision, the simulated images are three-dimensional and in some cases stereo sound that may also give a sense of motion. Studies have shown that exposure to rotational motions in a virtual environment can cause significant increases in nausea and other symptoms of motion sickness.