Momentum is the strength of motion that keeps something moving once it's already in motion. In physics, a rolling bowling ball has momentum: the heavier it is and the faster it rolls, the harder it becomes to stop. A bicycle coasting down a hill builds momentum with each passing second.

The word also describes a powerful force in human activities. When a basketball team wins three games in a row, they have momentum: their confidence grows, their teamwork sharpens, and they become harder to beat. A political campaign that starts winning supporters can gain momentum, attracting more and more people to its cause. Scientists working on a difficult problem might struggle at first, but once they make a breakthrough, their research gains momentum as each discovery leads to the next.

You can build momentum, gain momentum, or lose momentum. Starting a big project often feels hard because you haven't built any momentum yet. But once you get going and make real progress, momentum carries you forward. That's why coaches tell athletes to “keep the momentum going” after a good play, and why teachers encourage students to maintain momentum in their studies instead of constantly stopping and starting. Momentum turns effort into achievement by keeping energy flowing in the same direction.