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):

simultaneousness
    n 1: happening or existing or done at the same time [syn:
         simultaneity, simultaneousness]