Nitpick alert: You probably know this, but it’s an
important distinction that the non-plural usage of “data”
not only is grammatically singular, but is also a
mass noun.
(People say “I have some data, you have more data”, not *”I
have one data, you have two data[s]”.)
Datum is the neuter singular of the perfect passive participle of the Latin verb dare “to give.” This grammatical form is roughly analogous to the English participle “given.” However, in Latin, such participles are sometimes used as standalone nouns, so that the neuter form datum by itself can mean “[that which is/has been] given.” Analogously, the plural data can mean “[the things that are/have been] given.”
In English, this word has been borrowed with the meaning of “information given” and variations on that theme (besides a few additional obscure technical meanings).
Nitpick alert: You probably know this, but it’s an important distinction that the non-plural usage of “data” not only is grammatically singular, but is also a mass noun. (People say “I have some data, you have more data”, not *”I have one data, you have two data[s]”.)
Virtually everyone who makes “data” grammatically plural actually uses it as a mass noun, too.
...so what’s “datum”, then?
Datum is the neuter singular of the perfect passive participle of the Latin verb dare “to give.” This grammatical form is roughly analogous to the English participle “given.” However, in Latin, such participles are sometimes used as standalone nouns, so that the neuter form datum by itself can mean “[that which is/has been] given.” Analogously, the plural data can mean “[the things that are/have been] given.”
In English, this word has been borrowed with the meaning of “information given” and variations on that theme (besides a few additional obscure technical meanings).
It’s the singular that plural “data” is a plural of. Someone who strictly uses “data” as a mass noun would say “piece of data”.