The story
From the three dimensions visualized for the character space. The x-axis separates characters by semantics (function and meaning, e.g. A from B), the y-axis separates them by their abstract shape (general form, e.g. uppercase A from lowercase a), and the z-axis by their apparent shape (minor details such as typeface, e.g. printed from handwritten a).
Etymology from Wiktionary, CC BY-SA 4.0.