bidirectional interaction between humans and computers

A necessary but not base-line level of characteristic of virtual reality is causal-chain human-computer interaction. Human-to-computer communication can entail computer tracking of finger, hand, head, eye and/or body movement and/or voice recognition. Computer-to-human interaction may include three-dimensional image projections, audio production, and kinesthetic (touch and motion) simulation. Also, IEEE VR 2000 has information on this topic.

Computer-simulated things and worlds should conform with satisfactory accuracy to the tangible and other laws that shape such objects and environments in the tangible world. Some difference may be relevant for surreal applications, but in case the divergence is too large then the sense of realism is forfeit. Also, Virtual Tours Mechanicsville, Virginia deals with these concepts.

Simulated Robots discusses more about this and related topics.