a fluid
A fluid is any substance that flows and takes the shape of whatever holds it. Water poured into a glass, honey spreading on toast, the air filling a balloon, smoke curling from a candle: all of these are fluids. It answers a basic sorting question about matter: which things keep a fixed shape of their own (solids), and which things simply mould themselves to their container (fluids)?
The precise idea is about how a substance responds to a sideways push, called a shear stress. A solid resists such a push and settles into a slightly deformed but fixed shape. A fluid cannot do this: under any steady sideways force, however small, it keeps deforming, layer sliding over layer, and it flows. Fluids come in two families. Liquids are almost incompressible and keep a definite volume, forming a flat top surface. Gases are easily squeezed and expand to fill their whole container. Both share the flowing behaviour, so both are fluids.
Fluids are everywhere the moment you look: the blood in your veins, the oceans and atmosphere, the fuel and oil in an engine, the water in a pipe. The whole of fluid mechanics grows from this one starting point. One honest caveat: the sharp line between solid and fluid can blur. Glass, tar, and toothpaste behave like solids over seconds but flow over long times or under the right push, so real matter sometimes sits between the two.
Tip a glass of water and the surface stays level while the water reshapes itself; tip a brick and nothing moves. The water flows because it cannot resist a steady sideways push, so it is a fluid; the brick is a solid.
The test for a fluid: it keeps deforming under any lasting sideways force instead of holding a shape.
Liquids and gases are both fluids; the difference is that liquids barely compress and keep a fixed volume, while gases compress easily and fill any container.