The Collaborative International Dictionary of English v.0.48:

Simultaneous \Si`mul*ta"ne*ous\, a. [LL. simultim at the same
   time, fr. L. simul. See Simulate.]
   Existing, happening, or done, at the same time; as,
   simultaneous events. -- Si`mul*ta"ne*ous*ly, adv. --
   Si`mul*ta"ne*ous*ness, n.
   [1913 Webster]

   Simultaneous equations (Alg.), two or more equations in
      which the values of the unknown quantities entering them
      are the same at the same time in both or in all.
      [1913 Webster]

WordNet (r) 3.0 (2006):

simultaneously
    adv 1: at the same instant; "they spoke simultaneously" [syn:
           simultaneously, at the same time]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "simultaneously":
   all at once, all together, as one, as one man, at a blow,
   at a clip, at a stroke, at once, at one blow, at one jump,
   at one stroke, at one swoop, at one time, at that time,
   coincidentally, coinstantaneously, concurrently, contemporaneously,
   forthwith, in a chorus, in a hurry, in chorus, in concert with,
   in phase, in sync, in unison, isochronously, now, on that occasion,
   on the beat, per saltum, pronto, right away, right now, right off,
   straightaway, straightway, subito, synchronously, then,
   then and there, this minute, this very minute, together, uno saltu,
   with one voice, without delay