half-width character; single-byte character
Computing term for characters that occupy half the width of a full-width character, such as ASCII letters, digits, and single-byte kana. Often contrasted with 全角文字 (full-width character).
See also: 全角文字 · 半角カナ (はんかくカナ)
Please enter your password using half-width characters.
You need to distinguish between half-width and full-width characters.