The Collaborative International Dictionary
Plethrum
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.
Wiktionary
plethrum
n. (alternative form of plethron English)