The Collaborative International Dictionary of English v.0.48:

Imperishable \Im*per"ish*a*ble\, a. [Pref. im- not + perishable:
   cf. F. imp['e]rissable.]
   Not perishable; not subject to decay; indestructible;
   enduring permanently; as, an imperishable monument;
   imperishable renown. -- Im*per"ish*a*ble*ness, n. --
   Im*per"ish*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperishableness
    n 1: the property of being resistant to decay; "he advertised
         the imperishability of the product" [syn:
         imperishability, imperishableness, imperishingness]