The word pangram, though well-known to the logologist for many years, appears in few dictionaries. Such standard works as the Marriam-Webster Unabridged (both Second and Third Editions) and the Oxford English Dictionary omit it, and it is not mentioned in the Encyclopedia Britannica's long section on wordplay. The only dictionaries in which it appears at "6000 Words" (1976) and the third volume of the "Supplement to the Oxford English Dictionary" (1982). In both, the word is defined as a sentence containing all 26 letters of the alphabet. It is understood, although not explicitly specified, that some of these letters may appear more than once.
Word Ways: Vol. 20
, Article 11.
Available at: https://digitalcommons.butler.edu/wordways/vol20/iss2/11