A mixed reality is a combination of virtual, mostly computer-generated, information and the real world. Two special forms of the mixed reality are Augmented Reality and Augmented Virtuality. While Augmented Reality enriches the sense perceptions of reality with virtual information, Augmented Virtuality takes information from reality and maps it to a virtual world.
Mixed Reality is thus a more general term of these two special cases. It covers both these terms as well as all forms in between these special cases. However, Augmented Virtuality plays hardly a role at the moment. Therefore, some use augmented reality and mixed reality synonymous.