It's the shorthand/joking name we use for the set of basic documents. In the '51 world it happens that the manufacturers follow the way how Intel layed out the documents: as general "user manuals" (that's the "bible"), containing the "how does it work" information; plus the per-model "datasheets", containing electrical/timing data and information pertinent only to that given model.

Unfortunately, as Intel stopped manufacturing microcontrollers in 2007, they also gradually dropped the documentation from their website, so the "only true one" is to be found in archives today. But, as also mentioned many times, the Philips/NXP ones are almost verbatim copies and contrary to the scanned Intel ones, they are searchable (mostly) PDFs.

JW