The act of accessing shared computer resources without authorization. This implies the person or system acting has no authorization to access the resources.
Computer intrusion can happen to all sorts of computers, though it happens most to public and/or private servers--depending on the value of the resources shared.
If the unauthorized access requires intimate knowledge of the authorization challenge mechanism--perhaps to the point of requiring an artist or professional--such an artist would normally be labeled by modern society as a hacker. However, computer intrusion is not something one must do in order to be considered a hacker.