The Collaborative International Dictionary of English v.0.48:

Impermanent \Im*per"ma*nent\, a.
   Not permanent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impermanent
    adj 1: not permanent; not lasting; "politics is an impermanent
           factor of life"- James Thurber; "impermanent palm
           cottages"; "a temperary arrangement"; "temporary housing"
           [syn: impermanent, temporary] [ant: lasting,
           permanent]
    2: existing or enduring for a limited time only

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "impermanent":
   able to adapt, adaptable, adjustable, alterable, alterative,
   brittle, capricious, changeable, checkered, corruptible, deciduous,
   dying, ephemeral, evanescent, ever-changing, fading, fickle,
   fleeting, flexible, flitting, fluid, fly-by-night, flying, fragile,
   frail, fugacious, fugitive, impetuous, impulsive, inconstant,
   insubstantial, kaleidoscopic, malleable, many-sided, metamorphic,
   mobile, modifiable, momentary, mortal, movable, mutable,
   nondurable, nonpermanent, nonuniform, passing, perishable,
   permutable, plastic, protean, proteiform, resilient, rubbery,
   short-lived, supple, temporal, temporary, transient, transitive,
   transitory, undurable, unenduring, unstable, variable, volatile