Stream is flowing water moving continuously in one direction, usually in a natural channel. A stream is smaller than a river but bigger than a trickle. Picture water flowing over rocks in a forest, babbling and splashing as it makes its way downhill. Streams start high in mountains or hills and flow into larger bodies of water like rivers, lakes, or oceans.

When you read about someone going upstream, they're moving against the flow of the water, which takes extra effort. Going downstream means moving with the current, which is much easier. These ideas work as metaphors too: swimming upstream against popular opinion means working hard to resist what everyone else thinks.

The word also means a steady flow of anything continuous. You might hear a stream of complaints from a frustrated teammate, or watch a stream of ants marching toward a picnic blanket. When you stream a movie or show online, data flows continuously from the internet to your device, like water flowing from a source.

The word streamline means to make something more efficient by removing unnecessary parts or steps, like how water flows smoothly along a fish's sleek body. Teachers might streamline their classroom procedures to save time, or engineers might streamline a car's shape to reduce air resistance.