Multiple Master Fonts are an extension to the Adobe font format. providing the ability to interpolate smoothly between several ``design axes'' from a single font. Design axes can include weight, size, and even some whacko notions like serif to sans serif. Adobes' first Multiple Master Font was Myriad - a two-axis font with WEIGHT (light to black) on one axis, and WIDTH (condensed to expanded) along the other axis. In the case of Myriad, there are four ``polar'' designs at the ``corners'' of the design space. The four designs are light condensed, black condensed, light expanded, and black expanded.
Given polar designs, you can set up a ``weight vector'' which interpolates to any point within the design space to produce a unique font for a specific purpose. So you can get a ``more or less condensed, somewhat black face''.