An outline font (or "vector font") is one defined as vector graphics, i.e. as a set of lines and curves, as opposed to a bitmap font.
Examples are PostScript and TrueType.
Outline font characters can be scaled to any size and otherwise transformed more easily than a bitmap font, and with more attractive results, though this requires a lot of numerical processing. The result of transforming a character in an outline font in a particular way is often saved as a bitmap in a font cache to avoid repeating the calculations if that character is to be drawn again.
Article based on outline font at FOLDOC.
