To lengthen means to make something longer or to become longer. When you lengthen a rope, you add more rope to increase its length. When days lengthen in spring, the sun stays up longer each evening as summer approaches.

You can lengthen physical things: a tailor might lengthen the sleeves on a jacket that's become too short, or you might lengthen your stride when you're trying to walk faster. You can also lengthen time: a teacher might lengthen recess on a beautiful day, or you might lengthen your study session to prepare better for a test.

The word works in both directions. You might actively lengthen something yourself, like when you lengthen a story by adding more details. Or something might lengthen on its own, like how shadows lengthen as the sun moves lower in the sky.

The opposite of lengthen is shorten. If you need your essay to be longer, you lengthen it. If it's too long, you shorten it.