The Collaborative International Dictionary of English v.0.48:

Terce \Terce\, n.
   See Tierce.
   [1913 Webster]

WordNet (r) 3.0 (2006):

terce
    n 1: the third canonical hour; about 9 a.m. [syn: terce,
         tierce]

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

TERCE, law of Scotland. A life-rent competent by law to widows who have not
accepted of special provisions in the third part of the heritable subjects
in which the husband died infeft.
     2. The terce takes place only where the marriage has subsisted for a
year and day, or where a child has been born alive of it. No terce is due
out of lands in which the husband was not infeft, unless in case of a
fraudulent omission. Cr. 423, Sec. 28; St. 2, 6, 16. The terce is not
limited to lands, but extends to teinds, and to servitudes and other burdens
affecting lands. Ersk. Pr. L. Scot. B. 2, t. 9, s. 26, 27; Burge on the
Confl. of Laws, 429 to 435.