The Collaborative International Dictionary of English v.0.48:

Troth \Troth\, n. [A variant of truth. See Truth.]
   1. Belief; faith; fidelity.
      [1913 Webster]

            Bid her alight
            And hertroth plight.                  --Shak.
      [1913 Webster]

   2. Truth; verity; veracity; as, by my troth. --Shak.
      [1913 Webster]

            In troth, thou art able to instruct gray hairs.
                                                  --Addison.
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   3. Betrothal.
      [1913 Webster]

WordNet (r) 3.0 (2006):

troth
    n 1: a mutual promise to marry [syn: betrothal, troth,
         engagement]
    2: a solemn pledge of fidelity [syn: plight, troth]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "troth":
   adherence, adhesion, affiance, allegiance, assurance, assure,
   attachment, avouch, avouchment, banns, banns of matrimony,
   become engaged, betroth, betrothal, betrothing, betrothment,
   bona fides, bond, bonne foi, constancy, contract,
   contract an engagement, countersign, devotedness, devotion, engage,
   engagement, espousal, faith, faithfulness, fealty, fidelity,
   firmness, good faith, guarantee, handfasting, homage, loyalty,
   make a promise, marriage contract, oath, parole, pledge, plight,
   plighted faith, plighted troth, promise, publish the banns,
   solemn declaration, staunchness, steadfastness, swear, tie,
   true blue, trueness, underwrite, vouch, vow, warrant, warranty,
   word, word of honor