Dux

·noun The scholastic name for the theme or subject of a fugue, the answer being called the comes, or companion.

dux
dux, dŭcis, com. [duco], a leader, conductor, guide (for syn. cf.: imperator, ductor, tyrannus, rex,...

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.
dux
dux ducis, m and f DVC-, a leader, conductor, guide : itineris periculique, S.: locorum, L.: iis d...

An Elementary Latin Dictionary