Autochthonous: (Pronounced au-toch-tho-nous) 1. Native. Poison ivy is an autochthonous plant. 2. Found in the place where it formed. As an autochthonous thrombus. 3. From the same individual. As an autochthonous graft. Or autochthonous lymphocytes. Autochthonous is the adjective and autochthon is the noun. From auto + the Greek chthon, the earth, soil = sprung from the earth or the soil itself.