Dark Arts is a colloquial term for techniques or methods which involve deception and/or manipulation of others or oneself into believing things for non-truth-seeking reasons. These techniques may prey on human cognitive biases.
Some use the term to refer more narrowly to techniques that work equally well to compel both true and false beliefs, i.e., they are symmetric weapons. Some focus more on the Dark Arts as applied to oneself (self-deception) vs applied to manipulating others.
An example from the Dark Arts of Rationality:...