It is systematic pattern of deviation from norm or rationality in judgment. Individuals create their own subjective reality from their perception of the input, then it this "reality", not the objective input, that may dictate their behavior in the world. Thus, cognitive biases can oftentimes lead to perceptual distortion, inaccurate judgment, illogical interpretation, or what is broadly called irrationality. The biases you can discover on this site, together with their definitions, come directly from the Cognitive Bias Codex.