The Collaborative International Dictionary of English v.0.48:

Principle \Prin"ci*ple\, n. [F. principe, L. principium
   beginning, foundation, fr. princeps, -cipis. See Prince.]
   1. Beginning; commencement. [Obs.]
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            Doubting sad end of principle unsound. --Spenser.
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   2. A source, or origin; that from which anything proceeds;
      fundamental substance or energy; primordial substance;
      ultimate element, or cause.
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            The soul of man is an active principle. --Tillotson.
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   3. An original faculty or endowment.
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            Nature in your principles hath set [benignity].
                                                  --Chaucer.
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            Those active principles whose direct and ultimate
            object is the communication either of enjoyment or
            suffering.                            --Stewart.
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   4. A fundamental truth; a comprehensive law or doctrine, from
      which others are derived, or on which others are founded;
      a general truth; an elementary proposition; a maxim; an
      axiom; a postulate.
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            Therefore, leaving the principles of the doctrine of
            Christ, let us go on unto perfection. --Heb. vi. 1.
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            A good principle, not rightly understood, may prove
            as hurtful as a bad.                  --Milton.
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   5. A settled rule of action; a governing law of conduct; an
      opinion or belief which exercises a directing influence on
      the life and behavior; a rule (usually, a right rule) of
      conduct consistently directing one's actions; as, a person
      of no principle.
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            All kinds of dishonesty destroy our pretenses to an
            honest principle of mind.             --Law.
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   6. (Chem.) Any original inherent constituent which
      characterizes a substance, or gives it its essential
      properties, and which can usually be separated by
      analysis; -- applied especially to drugs, plant extracts,
      etc.
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            Cathartine is the bitter, purgative principle of
            senna.                                --Gregory.
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   Bitter principle, Principle of contradiction, etc. See
      under Bitter, Contradiction, etc.
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The Collaborative International Dictionary of English v.0.48:

Principle \Prin"ci*ple\, v. t. [imp. & p. p. Principled; p.
   pr. & vb. n. Principling.]
   To equip with principles; to establish, or fix, in certain
   principles; to impress with any tenet, or rule of conduct,
   good or ill.
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         Governors should be well principled.     --L'Estrange.
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         Let an enthusiast be principled that he or his teacher
         is inspired.                             --Locke.
   [1913 Webster] Princock

WordNet (r) 3.0 (2006):

principle
    n 1: a basic generalization that is accepted as true and that
         can be used as a basis for reasoning or conduct; "their
         principles of composition characterized all their works"
         [syn: principle, rule]
    2: a rule or standard especially of good behavior; "a man of
       principle"; "he will not violate his principles"
    3: a basic truth or law or assumption; "the principles of
       democracy"
    4: a rule or law concerning a natural phenomenon or the function
       of a complex system; "the principle of the conservation of
       mass"; "the principle of jet propulsion"; "the right-hand
       rule for inductive fields" [syn: principle, rule]
    5: rule of personal conduct [syn: principle, precept]
    6: (law) an explanation of the fundamental reasons (especially
       an explanation of the working of some device in terms of laws
       of nature); "the rationale for capital punishment"; "the
       principles of internal-combustion engines" [syn: rationale,
       principle]

Moby Thesaurus II by Grady Ward, 1.0:

204 Moby Thesaurus words for "principle":
   Procrustean law, a belief, a priori truth, activity, ambition,
   antecedents, article of faith, aspiration, at bottom, attitude,
   axiom, base, basement, basically, basis, bearing wall, bed,
   bedding, bedrock, belief, brocard, call, calling, campaign, canon,
   causation, cause, cause and effect, center, code, commandment,
   commitment, conscience, consideration, convention, core, credo,
   creed, criterion, crusade, determinant, determinative, dictate,
   dictum, doctrine, dogma, drive, element, elements, elixir, essence,
   essential, essentially, ethic, etiology, factor, faith, floor,
   flooring, flower, focus, fond, footing, form, formality, formula,
   formulary, foundation, fundament, fundamental, fundamentally,
   fundamentals, general principle, gist, given, goal, golden rule,
   grammar, gravamen, great cause, ground, grounds, groundwork, guide,
   guideline, guiding light, guiding principle, guiding star, hardpan,
   heart, honesty, hypostasis, idea, ideal, ideally, imperative,
   in essence, in theory, inner essence, inspiration, integrity,
   intention, interest, issue, kernel, law, law of nature, lifework,
   lodestar, mainspring, marrow, mass movement, matter, maxim, meat,
   mitzvah, model, moral, morality, morals, motive, movement, norm,
   norma, notion, nub, nucleus, nuts and bolts, occasion,
   order of nature, ordinance, pavement, philosophy, pith,
   point of view, postulate, precept, prescribed form, prescription,
   principium, principles, probity, proposition, quid, quiddity,
   quintessence, radical, reason, reason for being, regulation,
   riprap, rock bottom, rubric, rudiment, rudiments, rule, sake, sap,
   score, seat, self-evident truth, sentiment, set form,
   settled principle, sill, solid ground, solid rock, soul, source,
   spirit, spring, standard, standing order, stereobate, stimulus,
   stuff, stylobate, substance, substratum, substruction,
   substructure, teaching, tenet, terra firma, the nitty-gritty,
   theorem, theoretically, truism, truth, ulterior motive,
   underbuilding, undercarriage, undergirding, underpinning,
   understruction, understructure, universal law, universal truth,
   uprightness, usage, viewpoint, vocation, working principle,
   working rule



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PRINCIPLES. By this term is understood truths or propositions so clear that
they cannot be proved nor contradicted, unless by propositions which are
still clearer. They are of two kinds, one when the principle is universal,
and these are known as axioms or maxims; as, no one can transmit rights
which he has not; the accessory follows the principal, &c. The other class
are simply called first principles. These principles have known marks by
which they may always be recognized. These are, 1. That they are so clear
that they cannot be proved by anterior and more manifest truths. 2, That
they are almost universally received. 3. That they are so strongly impressed
on our minds that we conform ourselves to them, whatever may be our avowed
opinions.
     2. First principles have their source in the sentiment of our own
existence, and that which is in the nature of things. A principle of law is
a rule or axiom which is founded in the nature of the subject, and it exists
before it is expressed in the form of a rule. Domat, Lois Civiles, liv.
prel. t. 1, s. 2 Toull. tit. prel. n. 17. The right to defend one's self,
continues as long as an unjust attack, was a principle before it was ever
decides by a court, so that a court does Dot establish but recognize
principles of law.
     3. In physics, by principle is understood that which constitutes the
essence of a body, or its constituent parts. 8 T. R. 107. See 2 H. Bl. 478.
Taken in this sense, a principle cannot be patented; but when by the
principle of a machine is meant the modus operandi, the peculiar device or
manner of producing any given effect, the application of the principle may
be patented. 1 Mason, 470; 1 Gallis, 478; Fessend. on Pat. 130; Phil. on
Pat. 95, 101; Perpigna, Manuel des Inventeurs, &c., c. 2, s. 1.