The word digital literally comes from the Latin "digitus" meaning a finger or toe, which eventually entered English with both literal and figurative uses. By the 14th century, the word "digit" was used in English with the meaning of a natural and/or whole number up to ten through analogy (people would commonly count things and do minor arithmetic with their fingers). Quite literally, the original digital processing was done on people's fingers in prehistory, and then later in antiquity by written and mechanical devices that modeled fingers. When people counted with fingers, they regarded each of them as either up or down, which also means that digital data is inherently expressible in (or convertible to) a binary format!
The metaphor, then, is that "digital" data is data that can be counted on fingers, or, by extension, expressed exactly in numerals or another format that can be modeled losslessly by a sufficient number of fingers, each of them regarded as held either up or down (and not anywhere in between). That means that "digital" data is more or less synonymous with discrete data.
Examples of non-electronic digital data would thus include:
- Any data displayed by the holding up or down of various fingers
- Any data expressed exactly in numbers, such as 3, 55.2, or -0.99
- Data expressed in any form that has a lossless conversion back and forth to one of the above, such as:
- Data expressed on an abacus (an ancient digital mechanical calculator)
- Tally marks
- True/false or multiple-choice answers on a test
- Printouts of pre-calculated values, such as times tables, log tables, or trig tables
- Printed bus and train schedules (e.g. the R2 bus leaves Maple Park Square at 8:03 AM M-F heading northbound toward University Center East)
- Sheet music
- All forms of manual signboards that express or encode exact numbers, such as the scoreboard flipper mentioned by Steve
- Textual information that can be transcribed exactly into a word processor or text editor
Some of the categories above, of course, may have multiple encodings. For example, textual data may be encoded as ASCII, EBCDIC, or Unicode. The critical requirement is that at least one encoding exists which converts both back and forth without loss.
By contrast, the following data would not normally be considered digital:
- Positions on a ruler
- Positions on a slide rule
- Positions on a mercury thermometer, barometer, speedometer, or other physical analog gauge
- Most forms of graphic art
The above data and their associated devices are considered analog. The mechanical digital calculators mentioned by robert bristow-johnson would contrast with the analog slide rule.
Regarding the music angle, this corresponds nicely with digital versus analog music in the electronic world. A digital representation of music (e.g. an mp3 or a MIDI file) is nothing more than a numerical representation of how to play back the music, in other words, it is an electronic form of sheet music, providing discrete instructions to the computer or player on how to play it back.
Regarding written words and art, the difference is whether it is possible to transcribe exactly. The exact idiosyncrasies of the handwriting are irrelevant to the work. A handwritten novel, for example, can be converted to a text file without loss of any information considered relevant, since the exact form of the handwriting is considered outside of the scope of the novel's content. Calligraphy, however, involves appreciation of the relative positions of the loops, dots, and other forms made with the pen on paper (or other writing medium) and thus a computer transcript does not completely encode it.