Compared to movements elsewhere in Western Europe, the German Enlightenment is considered a latecomer and a fairly non-combative one at that. Using methods from the historiography of concepts, images, and metaphors, this article shows that a conception of Aufklärung as a form of intellectual improvement and a combative reform process emerged in the Germanspeaking lands shortly after 1700. It was decisive for the attractiveness of aufklären and Aufklärung that they found their origin in meteorological vocabulary; since the original meaning derives from the sun breaking through the clouds, the new cognitive meaning evoked visual associations that suggested an irresistible natural power of the desired reforms and a bright future. The study of the semantics and pragmatics of aufklären and Aufklärung spans from the early 17th to the mid-18th century. It is flanked by archive-supported investigations into the genesis of Geschichtliche Grundbegriffe, the major lexicon of historical concepts conceived and co-edited by Reinhart Koselleck. Against Koselleck’s thesis that history can only be understood in terms of concepts, this article works with a broader basis of sources, including pre-terminological and metaphorical language use as well as programmatic images. It also inquires into influences from other languages (French, English, Latin). Finally, the reported findings are compared with Dan Edelstein’s and Antoine Lilti’s interpretations of the Enlightenment.