The term ‘outline’ generally refers to the silhouette or a frame of the graphic design elements. The statement works for typefaces as well. Furthermore, the sans-serif outline fonts can be divided into subcategories. They may vary by several features, including some that:
are represented solely by a simple, laconic frame, where the insides of symbols are a blank transparent space (e.g., Elpida Thin Outline Font);
have a distinguished frame, yet the main body of letters are painted in another color (e.g., PopArt Vintage Typeface);
are executed as a common typeface, plus, each letter is once more outlined with the help of thin lines (e.g., Republiko – Display Typeface).