To undo means to reverse something that has been done, returning it to an earlier state. When you undo a knot in your shoelaces, you loosen and untangle it. When you undo the buttons on your coat, you unfasten them. The word suggests taking apart or canceling out an action.

In computing, the undo function lets you reverse your most recent action. If you accidentally delete a paragraph while writing a report, clicking undo brings it back. If you don't like the color you just added to your digital drawing, undo removes it. This feature has become so useful that people sometimes wish real life had an undo button.

The word can also describe reversing damage or harm. You might try to undo a mistake by apologizing and fixing what went wrong. Some mistakes are easy to undo, like erasing a pencil mark. Others are harder: if you accidentally tell someone's secret, you can't undo the fact that others now know it. That's why people sometimes say certain actions “can't be undone.”

The opposite of undo is do or redo. Many computer programs offer both undo and redo buttons, letting you move backward and forward through your changes until you get exactly what you want.