Definition of "Number"

In my opinion, a definition of 'number' which permits the inclusion of letters might look strange. One option is to simply to use the word 'series' without specifying the categories in the series.

The word can be used as follows:

The series 134432A contains a hidden message.

Having said that, there seems to be a variety of instances where the inclusion of non-numbers is acceptable. People use the term 'registration number' for a car (it often has letters), although I think the correct term is "registration plate". Your suggestion of an invoice number is a good example of where the inclusion of a letter might be possible. In other words, sometimes for convenience, the word 'number' is used even if a letter is included. However, I think that this usage might not be part of official definitions.

Yes, ish.

As BoldBen commented, the UK National Insurance Number is formatted AA 99 99 99 A (where A is letter and 9 is a decimal number).

There's also the Driver Number or Driving licence number on driving licences, format AAAAA 999999 AXXAA (where X is either a letter or the digit 9 as a space-filler).

There's an internationally-defined Vehicle Identification Number on vehicles, which contains letters and digits.

And the registration number of vehicles which displayed on their number plate consists of letters and digits.

Telephone numbers used to contain letters as well as digits. Serial numbers on equipment commonly contain letters as well as digits.

All of these are called numbers. However, all of them are qualified: "National Insurance" number; "Driver" number; "Vehicle Identification" Number; "registration" number; "telephone" number; "serial" number.

Dictionary definitions all refer to even serial numbers as numbers.

So: while a number needs to consist of digits, you can qualify a number for a particular purpose and that number (or what is called a number) might contain letters as well.

Digits compose numerals, which represent numbers. Eg, a hundred and one dots is the number of (non-gathered) dots represented by the numeral 101, with digits, 1, 0, and 1.

Possibly, the word alphanumeric came to be to describe stuff like alphanumeric problems, etc, which involve substituting numerical values for strings or sequences of letters, etc, with different digits for different letters.

I would call these things strings or sequences of characters containing such and such. Character string. Beyond this, a series is the sum, or product, etc, of a sequence of terms.

The term 'Number' is widely used sloppily for identifiers that can contain letters too

People have given you examples above, including:

• Passport numbers of some countries e.g. Germany (https://en.wikipedia.org/wiki/German_passport)
• Car Number Plates
• UK National Insurance Numbers
• Equipment Serial Numbers

This is now so widespread that we should probably now accept it as part of the language, and not argue with it.

'Alphanumeric' is not quite right

It satisfactorily describes the German passport "number", but not the codes you provided, where you (I assume) wanted all of these valid codes to be distinct.

134432A
134432-A
134432.A
134432 A

To take the extra effort to call your format an "alphanumeric" code, would be a shame, since a technically minded person (such as a person planning to store the data in a computer) might not allow for non-alpha, non-numeric symbols.

'Code' or 'Identifier'

These would be the simplest and yet accurate terms for what you want. If you think "Code" might sound too mysterious to normal people, "Identifier" should be more reassuring. People are already familiar with "ID" as a document of identification of a person, and would readily map that understanding to an "ID" being a unique text identifier of a thing.