The Collaborative International Dictionary of English v.0.48:

Tierce \Tierce\, n. [F. tierce a third, from tiers, tierce,
   third, fr. L. tertius the third; akin to tres three. See
   Third, Three, and cf. Terce, Tercet, Tertiary.]
   1. A cask whose content is one third of a pipe; that is,
      forty-two wine gallons; also, a liquid measure of
      forty-two wine, or thirty-five imperial, gallons.
      [1913 Webster]

   2. A cask larger than a barrel, and smaller than a hogshead
      or a puncheon, in which salt provisions, rice, etc., are
      packed for shipment.
      [1913 Webster]

   3. (Mus.) The third tone of the scale. See Mediant.
      [1913 Webster]

   4. A sequence of three playing cards of the same suit. Tierce
      of ace, king, queen, is called tierce-major.
      [1913 Webster]

   5. (Fencing) A position in thrusting or parrying in which the
      wrist and nails are turned downward.
      [1913 Webster]

   6. (R. C. Ch.) The third hour of the day, or nine a. m,; one
      of the canonical hours; also, the service appointed for
      that hour.
      [1913 Webster]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tierc'e \Tier*c['e]"\, a. [F.] (Her.)
   Divided into three equal parts of three different tinctures;
   -- said of an escutcheon.
   [1913 Webster] Tiercel

WordNet (r) 3.0 (2006):

tierce
    n 1: the third canonical hour; about 9 a.m. [syn: terce,
         tierce]
    2: the cardinal number that is the sum of one and one and one
       [syn: three, 3, III, trio, threesome, tierce,
       leash, troika, triad, trine, trinity, ternary,
       ternion, triplet, tercet, terzetto, trey, deuce-
       ace]
    3: one of three equal parts of a divisible whole; "it contains
       approximately a third of the minimum daily requirement" [syn:
       one-third, third, tierce]

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

TIERCE, measures. A liquid measure containing the third part of a pipe, or
forty-two gallons.