Simultaneity :: occurring, operating, or done at the same time : a simultaneous withdrawal of all troops | simultaneous translation. ORIGIN mid 17th cent.: based on Latin simul 'at the same time,' probably influenced by late Latin momentaneus. Simultaneous : adjective they carried out simultaneous raids at two houses concurrent, happening at the same time, contemporaneous, concomitant, coinciding, coincident, synchronous, synchronized.Photo © 2011 Ralph Lichtensteiger · All Rights Reserved.