Lias is a term used in geology to refer to a division or stage within the Jurassic period, a geological time period that occurred between approximately 201 million and 145 million years ago. It represents one of the three major divisions of the Jurassic, the other two being the Hettangian and the Toarcian.

The term Lias was originally derived from the village of Lyme Regis in England, where the rock strata of this Jurassic stage were well-exposed and studied extensively. The Lias is characterized by its distinctive sedimentary rock formations, primarily consisting of clays, limestones, and shales. Fossil records within the Lias provide valuable insights into the flora and fauna that thrived during this period, including various marine invertebrates, fish, and reptiles.

The Lias stage is divided into several sub-stages or zones based on the presence of specific ammonite species, which are useful indicators for stratigraphic correlation and age determination. Some notable ammonite genera found within the Lias include Dactylioceras, Echioceras, and Harpoceras.

Overall, the Lias represents an important time period in the Earth's history, particularly for paleontologists and geologists interested in understanding ancient ecosystems and the evolutionary trends of marine life during the Jurassic period.