In science, a fluid is any substance that flows and takes the shape of its container. Both liquids and gases are fluids. Water is a fluid because it flows and fills whatever container you pour it into, whether that's a glass, a bottle, or a river channel. Air is also a fluid, even though you can't see it. It flows around objects and fills any space available, which is why balloons expand to take a rounded shape and why wind can gust through an open window.

Understanding that gases are fluids helps explain many everyday phenomena. When you swim, you move through a liquid fluid. When you ride a bike, you push through a gaseous fluid (air). Both offer resistance, though air offers much less than water.

The word also means smooth and graceful, like flowing water. A fluid dancer moves without jerky motions, transitions flowing naturally from one position to the next. A fluid conversation moves easily from topic to topic without awkward pauses. When something is fluid, it adapts and changes smoothly rather than staying rigid or fixed. Plans might be fluid when they're still flexible and haven't been finalized yet.