Source : Webster's Revised Unabridged Dictionary (1913)

Rhumb \Rhumb\, n. [F. rumb, Sp. rumbo, or Pg. rumbo, rumo,
   probably fr. Gr. ??? a magic wheel, a whirling motion, hence
   applied to a point of the compass. See {Rhomb}.] (Navigation)
   A line which crosses successive meridians at a constant
   angle; -- called also {rhumb line}, and {loxodromic curve}.
   See {Loxodromic}.

   {To sail on a rhumb}, to sail continuously on one course,
      following a rhumb line.