What is Alphanumeric?

Legal Definition
Alphanumeric is a combination of alphabetic and numeric characters, and is used to describe the collection of Latin letters and Arabic digits or a text constructed from this collection.

Merriam-Webster observes that the term "alphanumeric" may often additionally refer to other symbols, such as punctuation and mathematical symbols.

In the POSIX/C locale, there are either 36 (A-Z+0-9, case insensitive) or 62 (A-Z+a-z+0-9, case-sensitive) alphanumeric characters.
-- Wikipedia
Legal Definition
Characters that can be letters or numbers.