:::UberCryxic, I've reverted your sources for three reasons: 
 ::: WP:RS states that "Wikipedia articles should be based on reliable secondary sources. This means that while primary or tertiary sources can be used to support specific statements, the bulk of the article should rely on secondary sources. Tertiary sources such as compendia, encyclopedias, textbooks, and other summarizing sources may be used to give overviews or summaries, but should not be used in place of secondary sources for detailed discussion." A dictionary should clearly not replace an academic study as a source as your edit did. 

 As stated in my edit summary, your three sources are out-of-date (1993-97): why would you replace an academic study from 2008 with a dictionary definition from 1993? Two of them are also North American where the term 'center-left' invariably has a different meaning never mind a different spelling to that in Britain.