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):

amortisation
    n 1: the reduction of the value of an asset by prorating its
         cost over a period of years [syn: amortization,
         amortisation]
    2: payment of an obligation in a series of installments or
       transfers [syn: amortization, amortisation]