A datum is a statement accepted at face value.
Data is the plural of datum.
A large class of practically important statements are measurements or observations of a variable.
Such statements may comprise numbers, words, or images.
The word data is the
plural of Latindatum, neuter past participle of dare, "to give",
hence "something given".
The past participle of "to give" has been used for millennia,
in the sense of a statement accepted at face value;
one of the works of Euclid, circa 300 BC,
was the Dedomena (in Latin, Data).
In discussions of problems in geometry, mathematics, engineering, and so on,
the terms givens and data are used interchangeably.
Such usage is the origin of data as a concept in computer science:
data are numbers, words, images, etc., accepted as they stand.
the word datum is still used in the general sense of "something given",
and more specifically in cartography, geography, and geology to mean a reference point, reference line, or reference surface.
The Latin plural data is also used as a plural in English,
but it is also commonly treated as a mass noun and used in the singular.
"This is all the data from the experiment".
This usage is inconsistent with the rules of Latin grammar,
which would suggest "These are the data ...",
each measurement or result being a single datum.
given the variety and irregularity of English plural constructions,
there seem to be no grounds for arguing that data is incorrect as a singular mass noun in English.
Raw data are numbers, characters, images or other outputs from devices to convert physical quantities into symbols,
in a very broad sense.
Such data are typically further processed by a human or input into a computer, stored and processed there, or transmitted (output) to another human or computer.
Raw data is a relative term; data processing commonly occurs by stages,
and the "processed data" from one stage may be considered the "raw data" of the next.
Mechanical computing devices are classified according to the means by which they represent data. An analog computer represents a datum as a voltage, distance, position, or other physical quantity. A digital computer represents a datum as a sequence of symbols drawn from a fixed alphabet. The most common digital computers use a binary alphabet, that is, an alphabet of two characters, typically denoted "0" and "1". More familiar representations, such as numbers and letters, are then constructed from the binary alphabet.
Some special forms of data are distinguished. A computer program is a collection of data which can be interpreted as instructions. Most computer languages make a distinction between programs and the other data on which programs operate, but in some languages, notably Lisp and similar languages, programs are essentially indistinguishable from other data. It is also useful to distinguish metadata, that is, a description of other data. The prototypical example of metadata is the library catalog, which is a description of the contents of books.