The Collaborative International Dictionary of English v.0.48:
Bimestrial \Bi*mes"tri*al\, a. [L. bimestris; bis twice + mensis month.] Continuing two months. [R.] [1913 Webster]WordNet (r) 3.0 (2006):
bimestrial adj 1: occurring every two months; "bimonthly visits" [syn: bimonthly, bimestrial] 2: two months long; lasting two months