Merriam-Webster Collegiate Dictionary - simultaneous

 adjective  Etymology: Latin simul at the same time + English -taneous (as in instantaneous) — more at same  Date: circa 1660  1. existing or occurring at the same time ; exactly coincident  2. satisfied by the same values of the variables ~ equations  Synonyms: see contemporary  • simultaneity noun  • ~ly adverb  • ~ness noun