noun
half-width digit
Technical term used in computing and typography to refer to digits that occupy half the width of a full-width character, such as ASCII digits (0-9).
パスワードには半角数字を必ず含めてください。
Please be sure to include half-width digits in your password.
Compound of 半角 (half-width) and 数字 (digit). The term is a modern technical coinage from computing and typography.