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):

simultaneous
    adj 1: occurring or operating at the same time; "a series of
           coincident events" [syn: coincident, coincidental,
           coinciding, concurrent, co-occurrent,
           cooccurring, simultaneous]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "simultaneous":
   accessory, accompanying, agreeing, associated, attendant,
   attending, coetaneous, coeternal, coeval, coexistent, coexisting,
   coincident, coinciding, coinstantaneous, collateral, combined,
   concomitant, concurrent, concurring, conjoint, contemporaneous,
   contemporary, conterminous, correlative, coterminous, coupled,
   fellow, isochronal, isochronous, joined, joint, mutual, paired,
   parallel, synchronal, synchronic, synchronous, twin, unison,
   unisonous