The term "fat lava" is believed to have started as an imprecise translation of "thick lava," describing the heavy, lava-like glaze on some West German pottery. However, it has evolved over time to be an accepted name for a certain type of West German pottery made from the 1950s to the 1970s, which may or may not have the lava glaze.
Mark Hill, the author of Fat Lava (West German Ceramics of the 1960s and 70s), is the expert most often mentioned on the sites I visited. Some of the leading manufacturers were Roth Keramik, Scheurich, Ruscha, Jasba, Dümler & Breiden and Bay.
From midcenturia.com, ginforodditiques.com, fat-lava.com, markhillpublishing.co.uk