Definition of lias in English:

1The earliest epoch of the Jurassic period, lasting from about 208 to 178 million years ago.

1. 1.1 The system of rocks deposited during the Lias, consisting of shales and limestones rich in fossils.
2. 1.2mass noun A blue-grey clayey limestone derived from marl deposited in the Lower Jurassic, found chiefly in south-western England.