The Collaborative International Dictionary of English v.0.48:

By-law \By"-law`\ (b[imac]"-l[add]`), n. [Cf. Sw. bylag, D.
   bylov, Icel. b[=y]arl["o]g, fr. Sw. & Dan. by town, Icel.
   b[ae]r, byr (fr. b[^u]a to dwell) + the word for law; hence,
   a law for one town, a special law. Cf. Birlaw and see
   Law.]
   1. A local or subordinate law; a private law or regulation
      made by a corporation for its own government.
      [1913 Webster]

            There was likewise a law to restrain the by-laws, or
            ordinances of corporations.           --Bacon.
      [1913 Webster]

            The law or institution; to which are added two
            by-laws, as a comment upon the general law.
                                                  --Addison.
      [1913 Webster]

   2. A law that is less important than a general law or
      constitutional provision, and subsidiary to it; a rule
      relating to a matter of detail; as, civic societies often
      adopt a constitution and by-laws for the government of
      their members. In this sense the word has probably been
      influenced by by, meaning secondary or aside.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bylaw
    n 1: a rule adopted by an organization in order to regulate its
         own affairs and the behavior of its members

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "bylaw":
   act, bill, canon, decree, dictate, dictation, edict, enactment,
   form, formality, formula, formulary, institution, jus, law,
   legislation, lex, measure, ordinance, ordonnance, prescript,
   prescription, regulation, rubric, rule, ruling, standing order,
   statute