Alphanumeric characters (letters and digits) displayed in half-width form in computing (e.g., A, 1), as opposed to full-width characters.
IDは半角英数字で入力してください。
Please enter the ID using half-width alphanumeric characters.
パスワード欄に全角が混ざっているとエラーになるので、半角英数字に直して再入力した。
Because having full-width characters in the password field caused an error, I converted them to half-width alphanumerics and re-entered it.