The prefix de- means to reverse an action, remove something, or reduce it. When you see de- at the start of a word, it often signals an undoing or taking away.

If you debug a computer program, you remove the bugs (errors) from it. When you deflate a balloon, you let the air out, reversing the inflation. To defrost a freezer means to remove the frost. A decaf coffee has had most of its caffeine taken away.

Sometimes de- means moving downward or away from something. When a plane descends, it moves down from a higher altitude. When you depart, you move away from where you were.

You'll find it in thousands of English words, from simple ones like detach (to unfasten or separate) to complex ones like deforestation (removing forests from land).

Not every word starting with “de-” uses this prefix, though. Words like decide, declare, and delicious have different origins, so the de- doesn't mean “undo” in those cases. But when you see de- in words about removing, reversing, reducing, or moving down or away, you've found the prefix at work.