The Collaborative International Dictionary of English v.0.48:

Momentary \Mo"men*ta*ry\, a. [L. momentarius. See Moment.]
   Done in a moment; continuing only a moment; lasting a very
   short time; as, a momentary pang.
   [1913 Webster]

         This momentary joy breeds months of pain. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

momentary
    adj 1: lasting for a markedly brief time; "a fleeting glance";
           "fugitive hours"; "rapid momentaneous association of
           things that meet and pass"; "a momentary glimpse" [syn:
           fleeting, fugitive, momentaneous, momentary]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "momentary":
   annual, biannual, biennial, bimonthly, biweekly, brief, brittle,
   capricious, catamenial, centenary, centennial, changeable,
   corruptible, daily, decennial, deciduous, diurnal, dying,
   ephemeral, evanescent, fading, fickle, fleeting, flitting,
   fly-by-night, flying, fortnightly, fragile, frail, fugacious,
   fugitive, hasty, hebdomadal, hourly, immediate, impermanent,
   impetuous, impulsive, inconstant, instant, instantaneous,
   insubstantial, lightning-like, menstrual, momentaneous, momently,
   monthly, mortal, mutable, nondurable, nonpermanent, passing,
   perishable, presto, quarterly, quick, quick as lightning,
   quick as thought, quotidian, secular, semestral, semiannual,
   semimonthly, semiweekly, semiyearly, short, short-lived, temporal,
   temporary, tertian, transient, transitive, transitory, triennial,
   undurable, unenduring, unstable, volatile, weekly, yearly