The Collaborative International Dictionary of English v.0.48:

Sutra \Su"tra\, n.; pl. Sutras. [Skr. s?tra a thread, a string
   of rules; an aphorism; fr. siv to sew.]
   1.
      (a) A precept; an aphorism; a brief rule.
      (b) A collection of such aphorisms.
          [1913 Webster]

   2. pl. A body of Hindoo literature containing aphorisms on
      grammar, meter, law, and philosophy, and forming a
      connecting link between the Vedic and later Sanscrit
      literature. --Balfour (Cyc. of India).
      [1913 Webster]

WordNet (r) 3.0 (2006):

sutra
    n 1: a rule or aphorism in Sanskrit literature or a group of
         aphoristic doctrinal summaries prepared for memorization

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "sutra":
   adage, ana, analects, aphorism, apothegm, axiom, byword, catchword,
   collected sayings, current saying, dictate, dictum, distich,
   epigram, expression, gnome, golden saying, maxim, moral, mot,
   motto, oracle, phrase, pithy saying, precept, prescript, proverb,
   proverbial saying, proverbs, saw, saying, sentence,
   sententious expression, sloka, stock saying, teaching, text, verse,
   wisdom, wisdom literature, wise saying, witticism, word,
   words of wisdom