Things that are simultaneous happen at exactly the same time — not one after the other, and not merely in the same afternoon. If the lights go on and the music starts in the same instant, those two events are simultaneous. A simultaneous translation is a voice in another language while the first speaker is still talking, not after they finish. In school maths, simultaneous equations are a set that are all true together, so you solve them as a group. The everyday phrase is “at the same time”; simultaneous is the more careful school-and-news word. Though actually, people do stretch it to mean “in the same period.” Strict speakers keep it for the same moment. The adverb is a different shape: simultaneously. “The two phones rang simultaneously.”