The Collaborative International Dictionary of English v.0.48:

Aorist \A"o*rist\ ([=a]"[-o]*r[i^]st), n. [Gr. 'ao`ristos
   indefinite; 'a priv. + "ori`zein to define, ? boundary,
   limit.] (Gram.)
   A tense in the Greek language, which expresses an action as
   completed in past time, but leaves it, in other respects,
   wholly indeterminate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aorist
    n 1: a verb tense in some languages (classical Greek and
         Sanskrit) expressing action (especially past action)
         without indicating its completion or continuation