If you are biased, you have an opinion or preference for something or someone. Usually the word is used when someone is not using reasoning, facts or being fair. For instance, a company may be biased against people from a certain background and not employ them. The company has a preference, BUT it is also not being fair or being reasonable. Sometimes the word is used only to simply say you have a preference for something. YOu can still be fair or reasonable. "I'm biased more towards eating apples than oranges". I am simply saying my preference is towards apples, and 'reasoning', 'the facts' have nothing to do with my opinion.

To favour one side more than another in any given situation. Example "He was biased towards French because his grandmother was a Parisian" Example "The BBC is currently accused of bias against the Labour Party"

Most basically explained, 'biased' means favouring/preferring one thing over another (e.g. biased toward optimism OR having a negative bias).