The Collaborative International Dictionary of English v.0.48:

Deciare \Dec"i*are`\, n. [F. d['e]ciare; pref. d['e]ci- tenth
   (fr. L. decimus) + are. See 2d Are.] (Metric System)
   A measure of area, the tenth part of an are; ten square
   meters.
   [Webster 1913 Suppl.]