Simultaneously means happening at exactly the same time. When you pat your head and rub your stomach simultaneously, both hands are moving at once. When the teacher asks everyone to stand up simultaneously, the whole class rises together in the same moment.

You'll see it used when timing matters: a conductor might want the entire orchestra to start playing simultaneously, or a coach might have all swimmers dive into the pool simultaneously at the sound of the starting buzzer.

Simultaneously is different from “consecutively” or “sequentially,” which mean one after another. If you read two books simultaneously, you're switching back and forth between them during the same time period. If you read them consecutively, you finish one completely before starting the next.

Scientists use this word often because precise timing matters in experiments. A solar eclipse happens when the moon passes between Earth and the sun, and observers in the right location see darkness fall simultaneously across a wide area. In everyday life, you might notice how hard it is to listen to someone and think about something else simultaneously: your brain can only focus on one thing at a time, even though you can do multiple physical actions at once.