"An ordered set of characters which can be combined to form the ensembles, words or expressions of a language or to represent data" (K. KRIPPENDORFF, 1986, p.2)
Many kinds of alphabets are in existence: the very numerous different alphabets of written and spoken languages, the conventional combinations based on a binary code, and even the 20 basic amino-acids of DNA and RNA chains.
To express that an alphabet is an "ordered set" means that it corresponds to differentiated symbols and that its use is constrained by defined rules. For example "chtch" is a sound and a letter in Russian and the combinations "mb", "ng", or "nz" are admitted in Kiswahili, but none of these are normally used in English.
Any alphabet is thus a specific system.
In a sense, any taxonomy that uses a kind of ordering base of classification to be applied on the combinations of a limited number of well defined elements, could be considered an alphabet.