Simultaneously means at exactly the same time. It describes two or more things happening together, not one after another.
“Simultaneously” comes from “simultaneous” plus the adverb suffix “-ly.” “Simultaneous” traces back to Latin “simul,” meaning together or at the same time. The adverb is common in formal and technical contexts where precise timing matters.
In everyday talk we say ‘at the same time’; simultaneously is the formal, Latinate version of that idea. It’s especially loved in science and engineering, where timing can’t be vague. Spotting this word in a sentence is a clue that coordination or cause-and-effect is important.
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