Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.

Source : WordNet®

simultaneousness
     n : happening or existing or done at the same time [syn: {simultaneity}]