The Collaborative International Dictionary of English v.0.48:

Gyron \Gy"ron\ (j[imac]"r[o^]n), n. [F. giron; of German origin.
   See Gore a piece of cloth.] (Her.)
   A subordinary of triangular form having one of its angles at
   the fess point and the opposite side at the edge of the
   escutcheon. When there is only one gyron on the shield it is
   bounded by two lines drawn from the fess point, one
   horizontally to the dexter side, and one to the dexter chief
   corner.
   [1913 Webster]