Bias is an inclination to present or hold a partial perspective at the expense of (possibly equally valid) alternatives. Bias can come in many forms.
A cognitive bias is the human tendency to make systematic decisions in certain circumstances based on cognitive factors rather than evidence. Such biases can result from information-processing shortcuts called heuristics. They include errors in judgment, social attribution, and memory.

"Both social and biosocial factors are necessary to interpret crosscultural studies, with the general proviso that one's research interest determines which elements, in what combinations, are significant for the provision of understanding."

Gilbert Herdt, "Bisexuality and the Causes of Homosexuality: The Case of the Sambia"

This is the essence of the problem. To Dan Rather and to a lot of other powerful members of the chattering class, that which is right of center is conservative. That which is left of center is middle of the road. No wonder they can't recognize their own bias.

Bernard Goldberg|Bernard Goldberg, Bias: A CBS Insider Exposes How the Media Distort the News|Bias: A CBS Insider Exposes How the Media Distort the News, 2001