Definitions of Received Pronunciation

http://en.wikipedia.org/wiki/Received_Pronunciation
Wikipedia defines the RP as a form of pronunciation of the English Language. It states that it is an accent, not a dialect, and through many contents, discuss its usage, how it has changed over time, the change of its status and very didactic charts of its vowels, consonants, diphthongs and triphthongs. The website also presents some characteristics, historical variation, some references and external links about the subject.

http://www.yaelf.com/rp.shtml
This website has an overview of the subject, in which it defines the RP as the prestigious accent of the well-educated people of the UK. It presents definitions by dictionaries, the etymology of the term “Received Pronunciation”, commentaries on the evolution and the speakers of the accent, on the difference between "BBC English", "The Queen's English", and "Public School English", on the distinguishing features of RP, and other useful information.

http://www.ic.arizona.edu/~lsp/RPEnglish.html
This simpler website presents a brief background of RP, also discussing the origins of the term “Received Pronunciation”, the geography of RP, in which it is said that it is not an accent of one specific region of Great Britain, explains the sociolinguistics issues of RP English, and has some examples of the Intrusive R of the accent.

http://www.bbc.co.uk/dna/h2g2/A852798
BBC’s websites presents an explanation about the origins of the RP, in which it claims that the Public Schools Systems of the 19th Century and the Ancient Universities (Oxford and Cambridge) are the ones who established it as standard. This text also explains the difference between “ultra-RP” and “marked-RP”; states the changes of the accent and describes who, nowadays, uses the RP English.

http://www.bl.uk/learning/langlit/sounds/case-studies/received-pronunciation/
The British Library defines the Received Pronunciation as a “social accent of English”. This site states that the RP is the most studied accent of English, but only 2% of the population speaks it. There is also an explanation of the name “Received Pronunciation”, an explanation of why it is the accent chosen by BBC, the difference between “Conservative RP”, “Mainstream RP” and “Contemporary RP” and a description of how is the RP today.

http://www.litnotes.co.uk/rphist.htm
This website defines RP as the accent associated with respectable social standing and a good education, presents some features of the accent and claims that it is the one used when teaching English to foreigners. The text also features the evolution of the RP, from before the 16th century until the 20th century, when broadcasting begun. It points the American influence as factor of dilution of the RP and relates that despite that, it still has status and survives in the 21st century.