Source : Webster's Revised Unabridged Dictionary (1913)

Bimestrial \Bi*mes"tri*al\, a. [L. bimestris; bis twice + mensis
   month.]
   Continuing two months. [R.]

Source : WordNet®

bimestrial
     adj 1: occurring every two months; "bimonthly visits" [syn: {bimonthly}]
     2: two months long; lasting two months