The fifth Harry Potter book – The Order of the Phoenix dedicates an entire chapter to the ancient branch of magic known as Occlumency. In chapter 24 of Harry Potter and the Order of the Phoenix, titled "Occlumency," Severus Snape pursues his despised student, Harry to Grimmauld Place to let him know that it is Headmaster Dumbledore's wish for him to take mandatory Occlumency lessons. Harry, unaware of the word, goes, "Study what?" and Snape replies, "Occlumency, Potter. The magical defense of the mind against external penetration. An obscure branch of magic, but a highly useful one." The professor's rather bookish description doesn't exactly help the young wizard as he wants to know why he needs defense against external penetration in the first place.
Harry Potter: Harry's Occlumency Lessons With Snape, Explained
