The Collaborative International Dictionary of English v.0.48:

Rule \Rule\, n. [OE. reule, riule, OF. riule, reule, F.
   r['e]gle, fr. L. regula a ruler, rule, model, fr. regere,
   rectum, to lead straight, to direct. See Right, a., and cf.
   Regular.]
   1. That which is prescribed or laid down as a guide for
      conduct or action; a governing direction for a specific
      purpose; an authoritative enactment; a regulation; a
      prescription; a precept; as, the rules of various
      societies; the rules governing a school; a rule of
      etiquette or propriety; the rules of cricket.
      [1913 Webster]

            We profess to have embraced a religion which
            contains the most exact rules for the government of
            our lives.                            --Tillotson.
      [1913 Webster]

   2. Hence:
      (a) Uniform or established course of things.
          [1913 Webster]

                'T is against the rule of nature. --Shak.
          [1913 Webster]
      (b) Systematic method or practice; as, my ule is to rise
          at six o'clock.
      (c) Ordibary course of procedure; usual way; comon state
          or condition of things; as, it is a rule to which
          there are many exeptions.
      (d) Conduct in general; behavior. [Obs.]
          [1913 Webster]

                This uncivil rule; she shall know of it. --Shak.
          [1913 Webster]

   3. The act of ruling; administration of law; government;
      empire; authority; control.
      [1913 Webster]

            Obey them that have the rule over you. --Heb. xiii.
                                                  17.
      [1913 Webster]

            His stern rule the groaning land obeyed. --Pope.
      [1913 Webster]

   4. (Law) An order regulating the practice of the courts, or
      an order made between parties to an action or a suit.
      --Wharton.
      [1913 Webster]

   5. (Math.) A determinate method prescribed for performing any
      operation and producing a certain result; as, a rule for
      extracting the cube root.
      [1913 Webster]

   6. (Gram.) A general principle concerning the formation or
      use of words, or a concise statement thereof; thus, it is
      a rule in England, that s or es, added to a noun in the
      singular number, forms the plural of that noun; but "man"
      forms its plural "men", and is an exception to the rule.
      [1913 Webster]

   7.
      (a) A straight strip of wood, metal, or the like, which
          serves as a guide in drawing a straight line; a ruler.
      (b) A measuring instrument consisting of a graduated bar
          of wood, ivory, metal, or the like, which is usually
          marked so as to show inches and fractions of an inch,
          and jointed so that it may be folded compactly.
          [1913 Webster]

                A judicious artist will use his eye, but he will
                trust only to his rule.           --South.
          [1913 Webster]

   8. (Print.)
      (a) A thin plate of metal (usually brass) of the same
          height as the type, and used for printing lines, as
          between columns on the same page, or in tabular work.
      (b) A composing rule. See under Conposing.
          [1913 Webster]

   As a rule, as a general thing; in the main; usually; as, he
      behaves well, as a rule.

   Board rule, Caliber rule, etc. See under Board,
      Caliber, etc.

   Rule joint, a knuckle joint having shoulders that abut when
      the connected pieces come in line with each other, and
      thus permit folding in one direction only.

   Rule of the road (Law), any of the various regulations
      imposed upon travelers by land or water for their mutual
      convenience or safety. In the United States it is a rule
      of the road that land travelers passing in opposite
      directions shall turn out each to his own right, and
      generally that overtaking persons or vehicles shall turn
      out to the left; in England the rule for vehicles (but not
      for pedestrians) is the opposite of this.

   Rule of three (Arith.), that rule which directs, when three
      terms are given, how to find a fourth, which shall have
      the same ratio to the third term as the second has to the
      first; proportion. See Proportion, 5
      (b) .

   Rule of thumb, any rude process or operation, like that of
      using the thumb as a rule in measuring; hence, judgment
      and practical experience as distinguished from scientific
      knowledge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rule of thumb
    n 1: a rule or principle that provides guidance to appropriate
         behavior [syn: guidepost, guideline, rule of thumb]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "rule of thumb":
   R and D, control, control experiment, controlled experiment,
   cut and try, empiricism, experiment, experimental design,
   experimental method, experimental proof, experimentalism,
   experimentation, hit and miss, noble experiment, pragmatism,
   research and development, tentative method, tentativeness, testing,
   trial, trial and error, trying