Of, relating to, or designating the geologic time, system of rocks, and sedimentary deposits of the second period of the Paleozoic Era, characterized by the appearance of primitive fishes.

• 奥陶纪（系）的属于、有关或指奥陶纪（系）的，奥陶纪（系）是指古生代第二纪的一段地质时间、岩石系统及沉积物，该纪以早期鱼的出现为特征