half-width Roman character
Refers to single-byte alphanumeric characters as opposed to full-width (全角) ones, commonly used in computing and typing contexts.
Please enter your password using half-width Roman characters.
半角英数字 includes both half-width alphabetic and numeric characters, whereas 半角ローマ字 specifically refers to Roman letters.
Compound of 半角 (half-width) and ローマ字 (Roman characters). The term is a modern technical coinage from computing.