Gaseous means existing as a gas, one of the three main states of matter (along with solid and liquid). When something is gaseous, its particles spread out to fill whatever container holds them, like how steam from a hot shower fills your bathroom or how the smell of cookies baking drifts through your whole house.

Water provides a perfect example of how matter changes states: ice is the solid form, liquid water is the liquid form, and steam or water vapor is the gaseous form. The sun is mostly made of gaseous hydrogen and helium. The air you breathe is a gaseous mixture of nitrogen, oxygen, and other gases.

Scientists use this word when they want to be precise about something being in gas form. A chemist might say that chlorine becomes gaseous at room temperature, or that carbon dioxide is released in its gaseous state when you open a soda bottle.