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: Webster's Revised Unabridged Dictionary (1913)