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.