HeidelbergPronunciation: /ˈhʌɪd(ə)lbəːg, ˈhaɪdlbɛrk/Definition of Heidelberg a city in SW Germany, on the River Neckar in Baden-Württemberg; population 144,600 (est. 2006). Its university is the oldest in Germany. Heidelberg in other Oxford dictionaries Definition of Heidelberg in the US English dictionary