| Definition -
The regionally-neutral British-English prestige accent.
1. It is an accent (i.e. a form of pronunciation), not a dialect (a vocabulary and rules of grammar).
2. About two percent of Britons speak its pure form.
Etymology - The term was coined by the linguist A J Ellis in 1869. However, it became widely used only after the phonetician Daniel Jones used it in the second edition of his English Pronouncing Dictionary (1924). Ultimately, it probably received its greatest acceptance when Lord Reith, the first General Manager of the BBC, adopted it in 1922 as a broadcasting standard; hence, the origin of the term BBC English.
(Source: the British Library website)
Oxford English Dictionary - Its first citation is from 1818:
"According to its most generally received pronunciation, it is more properly a diphthong."
Note: This quote doesn't seem quite right, but it is the first one the OED offers up.
(1818 Trans. Amer. Philos. Soc. I. 259)