The Collaborative International Dictionary of English v.0.48:

Lynch law \Lynch" law`\n.
   The act or practice by private persons of inflicting
   punishment for crimes or offenses, without due process of
   law.
   [1913 Webster]

   Note: The term Lynch law is said to be derived from a
         Virginian named Lynch, who took the law into his own
         hands. But the origin of the term is very doubtful.
         [1913 Webster] Lynde

WordNet (r) 3.0 (2006):

lynch law
    n 1: the practice of punishing people by hanging without due
         process of law