The megas doux was one of the highest positions in the hierarchy of the later Byzantine Empire, denoting the commander-in-chief of the Byzantine navy. It is sometimes also given in English by the half-Latinizations megaduke or megadux. The Greek word δούξ is the Hellenized form of the Latin term dux, meaning leader or commander.

Grand duke is a European hereditary title, used either by certain monarchs or by members of certain monarchs' families. In status, a grand duke traditionally ranks in order of precedence below an emperor, King, grand prince, archduke, or prince-archbishop, and above a sovereign prince or sovereign duke. The title is used in some current and former independent monarchies in Europe, particularly:in the present-day Grand Duchy of Luxembourg historically by the sovereigns of former independent countries, such as Tuscany in Baden, Hesse, Oldenburg, Mecklenburg-Schwerin, Mecklenburg-Strelitz and Saxe-Weimar – grand duchies from 1815 to 1918, and all now part of present-day Germany formerly also in some countries in Northern Europe, such as the Grand Duchy of Finland or the Grand Duchy of Lithuania