half-width alphanumeric characters
Refers to single-byte letters and digits used in computing contexts, as opposed to full-width (全角) characters.
Please enter your password using half-width alphanumeric characters.
全角英数字 are full-width alphanumeric characters, which take up twice the horizontal space of 半角英数字 and are often used in Japanese text for aesthetic or compatibility reasons.
Compound of 半角 (half-width), 英 (English/alphabetic), and 数字 (numerals). The term is a technical computing expression with no deeper historical derivation.