The ability to read and/or influence other people's impressions, thoughts, attitudes, and their knowledge. It is unsure whether or not a willingness to have their thoughts read is required from the intended subject, due to the current biases of the scientific community, and their unwillingness to investigate these phenomenea/abilities further.
However, it is not clear that that further scientific investigation would prove anything. The published literature of psychology contains studies that were conducted (many in the 1960s) in an attempt to demonstrate telepathy. A few suggestive positive results were obtained (example), but most scientists have concluded that belief in telepathy rests on erroneous interpretations of conventional psychological processes such as empathy and charisma. It may be that mind reading is possible because people are able to correctly predict (by using conventional thought processes) what other people are thinking. It is fairly simple o conduct research on telepathy. If anyone feels that the scientific community has not properly explored telepathy, they are free to conduct additional experiments.