The Collaborative International Dictionary of English v.0.48:

Detour \De`tour"\, n. [F. d['e]tour, fr. d['e]tourner to turn
   aside; pref. d['e]- (L. dis-) + tourner to turn. See Turn.]
   A turning; a circuitous route; a deviation from a direct
   course; as, the detours of the Mississippi.
   [1913 Webster]

WordNet (r) 3.0 (2006):

detour
    n 1: a roundabout road (especially one that is used temporarily
         while a main route is blocked) [syn: detour, roundabout
         way]
    v 1: travel via a detour