The Collaborative International Dictionary of English v.0.48:

Crith \Crith\ (kr[i^]th), n. [Gr. kriqh` a barleycorn, a small
   weight.] (Chem.)
   The unit for estimating the weight of a["e]riform substances;
   -- the weight of a liter of hydrogen at 0[deg] centigrade,
   and with a tension of 76 centimeters of mercury. It is 0.0896
   of a gram, or 1.38274 grains.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crith
    n 1: the weight of a liter of hydrogen (at 0 centigrade and 760
         millimeters pressure)