By definition, the word bias refers to showing an unjustified favoritism toward something or someone. Thus, on a very simplistic level, media bias refers to the media exhibiting an unjustifiable favoritism as they cover the news. When the media transmit biased news reports, those reports present viewers with an inaccurate, unbalanced, and/or unfair view of the world around them.
Political communication scholars have identified and examined two main types of bias in media reporting. The first type, commonly referred to as “partisan bias,” involves media reports that are slanted in favor of a particular political party. The second type of media bias is known as “structural bias.” This type of bias stems from certain “structures” (customs, reporting routines, commercial pressures, etc.) that operate within the news industry.
Levasseur, David, G. “Media Bias.” Encyclopedia of Political Communication, by Lynda Lee Kaid, Sage Publications, 2008. Credo Reference, https://search.credoreference.com/articles/Qm9va0FydGljbGU6OTEzMTI0?summaryArticle=true&aid=98275.