A preference in favor of, or against a person, group of people, or thing. These initial human reactions, which are often unconscious, are rooted in untrue information and/or cultural lore or structural defaults. These cognitive shortcuts can result in prejudgments that lead to rash decisions or discriminatory practices.
Biases are also part of being human. Once we know and accept we have biases, with awareness and a conscious effort, we have the power to change how we think and act.
See Implicit Bias or Stereotype Threat for additional examples.