The Collaborative International Dictionary of English v.0.48:

Towline \Tow"line`\, n. [AS. tohline. See Tow, v. t., and
   Line. ] (Naut.)
   A line used to tow vessels; a towrope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

towline
    n 1: (nautical) a rope used in towing [syn: towline,
         towrope, towing line, towing rope]