The Collaborative International Dictionary of English v.0.48:

Plethron \Pleth"ron\, Plethrum \Pleth"rum\, n.; pl. Plethra.
   [NL., fr. Gr. ?.] (Gr. Antiq.)
   A long measure of 100 Greek, or 101 English, feet; also, a
   square measure of 10,000 Greek feet.
   [1913 Webster]