Occlumency

Occlumency is the discipline of closing one’s mind against Legilimency, the reading of thoughts. After Harry’s visions of the corridor and of Arthur Weasley’s attack, Dumbledore orders lessons with Snape. Harry dislikes the intrusion and does not master it. The unread Occlumency is part of why he believes the vision of Sirius in the Department of Mysteries.