Occlumency
Occlumency is, in the fictional realm of J. K. Rowling's Harry Potter series, a branch of magic involving the practice of closing one's mind against external penetration, with the intent of preventing others from reading one's thoughts. One who is practiced in occlumency is known as an Occlumens. The companion art to Occlumency is that of Legilimency, the ability to read another person´s mind. Severus Snape and Albus Dumbledore are experts in the field.
The term "Occlumens" was created by J. K. Rowling through the combination of the word "occlude" (to close or block off) and the suffix "-mens," (which means mind).
Death Eater Severus Snape apparently uses Occlumency to lie to Lord Voldemort about the fact that he is spying on Voldemort for the Order of the Phoenix, although some interpret the events in Harry Potter and the Half-Blood Prince to mean that he was never truly a loyal member of the Order, and rather a double agent working for Voldemort. It is still debatable, what side he is using Occlumency against.
In Harry Potter and the Order of the Phoenix, Snape attempts to teach student Harry Potter occlumency by orders of Albus Dumbledore in order to defend himself from being manipulated by the evil Lord Voldemort. Dumbledore is an expert Occlumens as well. Harry is taught to clear his thoughts and close his mind before going to bed. This is advice he neglects, so that he can continue having his dreams about The Department of Mysteries. Eventually, Snape stops teaching Occlumency due to the fact that Harry looked at a pensieve of Snape's worst memory.
Later, Dumbledore would regret not teaching Harry Occlumency himself. This is because Snape and Harry were unable to make much progress due to their mutual loathing for each other.