This is how the Merriam-Webster Online Dictionary defines bias:
BENT , TENDENCY: an inclination of temperament or outlook; especially: a personal and sometimes unreasoned judgment: PREJUDICE: an instance of such prejudice i.e.,(1): deviation of the expected value of a statistical estimate from the quantity it estimates (2): systematic error introduced into sampling or testing by selecting or encouraging one outcome or answer over others.
or from Roget's II: The New Thesaurus, Third Edition:
Exhibiting bias: biased, one-sided, partisan, prejudiced, prejudicial, prepossessed, tendentious. See LIKE, STRAIGHT. 2. Disposed to favor one over another: favorable, preferential.
When it comes to modern media like news, advertising and the Internet, bias on demand has become the profession of spin-doctors who specialize in spinning anything negative into something positive.
Why should we check the bias of information on a web page?
Would you trust information unsupported by facts or logical reasoning? A biased author may not pay attention to all the facts or develop a logical argument to support his or her opinions.
Bias is when a statement reflects a partiality, preference, or prejudice for or against a person, object, or idea. Much of what you read and hear expresses a bias. Bias is when a writer or speaker uses a selection of facts, choice of words, and the quality and tone of description, to convey a particular feeling or attitude. Its purpose is to convey a certain attitude or point of view toward the subject. As you read or listen to biased materials, keep the following questions in mind:
What facts has the author omitted?
What additional information is necessary?
What words create positive or negative impressions?
What impression would I have if different words had been used?