<character> A decimal digit or a letter (upper or lower case). Typically, "letters" means only English letters (ASCII A-Z plus a-z) but it may also include non-English letters in the Roman alphabet, e.g., e-acute, c-cedilla, the thorn letter, and so on. Perversely, it may also include the underscore character in some contexts.
Try this search on Wikipedia, OneLook, Google
Nearby terms: alpha conversion « Alpha EV6 « Alpha Geek « alphanumeric » alpha particle » Alphard » alpha testing