A myth invented in conservative think-tanks as part of an all-encompassing effort to co-opt and/or corrupt all of the best ideas of a socially responsible, free country: In this case specifically, the existence of a free and curious media that functions independently from the government (but see also: affirmative action, feminism, social security, constitutional protection, judicial restraint, environmentalism, and social liberalism in general).
"Thank God we have Bill O'Reilly and Fox News to tell us the news as President Bush would have us know it -- before it gets filtered through the liberal bias for that pesky 'truth' the mainstream media is always harping on.
2. A one-liner, manufactured pejorative term like "socialist", that North American conservative Elites have created to justify the funding of blatantly biased, inaccurate news (ie. Fox News).
I hate how nature has a liberal bias! Every time I see a mother breastfeed her child I want to puke at nature's maternal welfare system which only serves to create laziness in human babies.
I'm not saying liberal bias is a bad thing.
For numerous examples of a liberal bias turn on NBC, ABC, CBS, CNN, MSNBC, PBS, NPR, etc.