To decelerate is to slow down or reduce speed.
It combines the prefix “de-,” meaning “down” or “away,” with “accelerate,” which comes from Latin “accelerare,” “to hasten.” So it literally means “to un-hasten” or “move away from speeding up.”
“Decelerate” is just “accelerate” in reverse—same root, opposite direction. Seeing how prefixes flip meanings like this helps you decode lots of technical-sounding words without memorizing them.
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