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

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