The Collaborative International Dictionary of English v.0.48:

Instantly \In"stant*ly\, adv.
   1. Without the least delay or interval; at once; immediately.
      --Macaulay.
      [1913 Webster]

   2. With urgency or importunity; earnestly; pressingly. "They
      besought him instantly." --Luke vii. 4.

   Syn: Directly; immediately; at once. See Directly.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instantly
    adv 1: without delay or hesitation; with no time intervening;
           "he answered immediately"; "found an answer
           straightaway"; "an official accused of dishonesty should
           be suspended forthwith"; "Come here now!" [syn:
           immediately, instantly, straightaway, straight
           off, directly, now, right away, at once,
           forthwith, like a shot]
    2: without any delay; "he was killed outright" [syn:
       instantaneously, outright, instantly, in a flash]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "instantly":
   PDQ, amain, apace, at once, away, by forced marches, cursorily,
   decisively, directly, expeditiously, feverishly, forthwith,
   furiously, hand over fist, hastily, helter-skelter, hotfoot,
   hurriedly, hurry-scurry, immediately, in no time, in passing,
   instantaneously, instanter, just now, just then, momentaneously,
   momentarily, momently, now, on the dot, on the instant,
   on the nail, on the run, on the spot, pell-mell,
   pretty damned quick, promptly, pronto, quickly, right, right away,
   right off, slapdash, smartly, speedily, straightaway, straightway,
   subito, summarily, superficially, swiftly, with a rush,
   with all haste, with all speed, with dispatch, with haste,
   without delay, without further delay