A departure from impartiality in cognition. This notion is distinct from concepts of error in morality or ethics. Being a bias, we’d expect to get approximately the same wrong answer every time a specific (cognitive) process is executed on the same set of inputs.
Several types of cognitive bias have been identified, all of which are heuristic, and some of which overlap. No universal, empirical list should be expected anytime soon, but the concept remains very useful.
See also the Bias Mitigation Exegesis.