The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   To sail on a rhumb, to sail continuously on one course,
      following a rhumb line.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rhumb
    n 1: a line on a sphere that cuts all meridians at the same
         angle; the path taken by a ship or plane that maintains a
         constant compass direction [syn: rhumb line, rhumb,
         loxodrome]