Alphanumeric means consisting of both letters and numbers, and in some definitions it can also include other symbols such as punctuation marks and mathematical symbols. An alphanumeric character is a character used in that kind of system—typically the letters A–Z (and a–z) plus the digits 0–9.
The term is common in computing and everyday codes: passwords, license plates, and character encodings such as ASCII are often described as alphanumeric because they mix letters and numerals (and sometimes additional symbols).
Source: Merriam-Webster — alphanumeric.