Something given as a standard: datum- line, the base- line from which surface levels are reckoned.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n. sing.) (noun singular)
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Something given or admitted; a quantity or fact given, known, or admitted, by which things or results unknown may be found.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (s. & pl.) (singular, plural)
Something assumed, known, or granted for the basis of an argument or inference; usually in plural.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)

Usage examples:

But, just as in the case of all the other sciences, when her investigations have been pushed to the point where they encounter the problem of explaining this principle itself, her investigations must necessarily cease; this principle is for all the sciences the ultimate datum behind which they cannot go without ceasing to be sciences.
- Mind and Motion and Monism by George John Romanes
Agri capti septena iugera populo viritim divisit; cumque ipsi senatus iugera quinquaginta adsignaret, plus accipere noluit quam singulis civibus erat datum dixitque perniciosum esse civem, qui eo, quod reliquis tribueretur, contentus non esset.
- Selections from Viri Romae by Charles Fran├žois L'Homond
I was using your disappearance as a datum in a problem that didn't require it.
- Dead Giveaway by Gordon Randall Garrett