double-width character; double-size character
Computing term for a character that occupies twice the normal width and/or height, often used in text formatting or display settings.
In this software, you can emphasize titles using double-width characters.
Double-width characters are displayed at twice the width of normal characters.
半角文字 is a half-width character, occupying half the width of a full-width character, while 倍角文字 is a double-width character, occupying twice the width.
全角文字 is a full-width character, the standard width for CJK characters, while 倍角文字 is a double-width character, even wider.
Compound of 倍 (double) + 角 (square, here referring to the em-square of a character) + 文字 (character). The term is a technical coinage from computing.