The Collaborative International Dictionary of English v.0.48:

Amortise \A*mor"tise\, v., Amortisation \A*mor`ti*sa"tion\, n.,
Amortisable \A*mor"tis*a*ble\, a., Amortisement
\A*mor"tise*ment\, n.
   Same as Amortize, Amortization, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amortise
    v 1: liquidate gradually [syn: amortize, amortise]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AMORTISE, contracts. To alien lands in mortmain.