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.