While they didn't create the term, TV Tropes defines "Mind Rape" as when "a character is attacked by a villain in the most painful non-physical way possible. Their mind and soul are assaulted with painful, horrifying visions and memories, and broken until they're powerless and numb, but not dead, although afterwards they may wish they were. Nothing sexual occurs, but everything else is there to resemble a rape - violation, helplessness, and the poisoning of what could otherwise be a source of joy."
However I've seen a lot of people throw the term around whenever a character gets their mind read without their permission, as if it's just as bad (or almost as bad) as the act of sexual assault itself.
One such example occurred in New Avengers #19, by Brian Michael Bendis and Mike Deodato.
SHIELD has asked the New Avengers for help dealing with the Collective, which is later revealed to be the unified energy signatures of all the mutants depowered after "M-Day." Spider-Man and the Young Avengers' Vision are on the Helicarrier when SHIELD discovers that the energy readings match those of a large number of the depowered mutants. When Spider-Man discovers the connection to the House of M, Iron Man tells him to take the Vision and get off the ship.
Another example of when the term "mind rape" was used was in reference tothis scene from Captain America #28, by Ed Brubaker and Steve Epting.
So, do you think any of these examples are comparable to actually raping someone?