The title of this node is very confusing. An "alphabet" is a specific set of characters used for writing natural language, like the Latin alphabet or the Hebrew alphabet (whence the term). I believe the word you're looking for is "character" or "letter": "t" is a character which is a letter (letter being a specific class of written character).

In short, a character is the atomical unit of a string. Characters comonly include digits or "numerical digits" (eg, 1, 2, 3), letters (eg, 'a', 'b', 'ɣ'), punctuation (eg, ':', '#', '!'). Letters and numbers taken together are called "alphanumeric characters", or sometimes "alphanumerics".

Note that this is an issue of natural language, not of programming. Specifically, do not try to map these classes of characters to regular expression character classes without consulting the relevant documentation, especially for the purposes of input validation.