Fürst
Fürst (German pronunciation: [ˈfʏʁst] ⓘ, female form Fürstin, plural Fürsten; from Old High German furisto, "the first", a translation of the Latin princeps) is a German word for a ruler as well as a princely title. Fürsten were, starting in the Middle Ages, members of the highest nobility who ruled over states of the Holy Roman Empire and later its former territories, below the ruling Kaiser (emperor) or König (king).
A prince of the Holy Roman Empire was the sovereign ruler of an imperial estate that held imperial immediacy in the boundaries of the Holy Roman Empire. The territory ruled is referred to in German as a Fürstentum (principality), the family dynasty referred to as a Fürstenhaus (princely house), and the (non-reigning) descendants of a Fürst are titled and referred to in German as Prinz (prince) or Prinzessin (princess).
The English language uses the term "prince" for both concepts. Latin-based languages (French, Italian, Romanian, Spanish, Portuguese) also employ a single term, whereas Dutch as well as the Scandinavian and some Slavic languages use separate terms similar to those used in German (see Knyaz for the latter).
An East-Asian parallel to the concept of "ruling prince" would be the Sino-Xenic word 王 (pronounced wáng in Mandarin, wong4 in Cantonese, ō in Japanese, wang in Korean, and vương in Vietnamese), which commonly refers to Korean and non-East-Asian "kings", but usually means non-imperial monarchs (who would go by 皇帝 ("emperor") instead) in ancient China, and therefore is frequently translated to "prince", especially for those who became rulers well after to the first adoption of the title 皇帝 by Qin Shi Huang. On the other hand, the son of a monarch would go by different titles, such as 皇子 ("imperial son") or 王子 ("royal son").
From the Middle Ages on, the German designation and title of Fürst referred to: