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.
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   2. Truth; verity; veracity; as, by my troth. --Shak.
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            In troth, thou art able to instruct gray hairs.
                                                  --Addison.
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   3. Betrothal.
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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]