To slacken means to become looser, slower, or less intense. When you slacken your grip on a rope, you relax your fingers and let it hang less tightly. When a runner slackens their pace near the end of a race, they slow down because they're tired or feel confident about finishing.

The word often describes something that was once tight, fast, or strong becoming more relaxed or reduced. A taut rope slackens when you stop pulling it. A strong wind might slacken to a gentle breeze. After weeks of intense studying, your effort might slacken once the big test is over.

Slacken can describe physical things like loosening a belt or easing up on bicycle brakes, but it also works for abstract ideas. Demand for a popular toy might slacken after the holidays. A student's attention might slacken during a long lecture on a hot afternoon.

The word carries a neutral tone: sometimes slackening is good (like when painful muscle tension slackens after stretching), and sometimes it signals a problem (like when your focus slackens during an important task). Notice that slacken and slack off are related but different: to slack off specifically means to become lazy or stop trying as hard as you should.