Matter
Definition
Anything that has mass and occupies space. Your desk, the air in the room, and a drop of water are all matter.
Technical definition
Matter is anything that possesses mass and volume, existing as solids, liquids, gases, or plasmas, and is composed of atoms, molecules, or ions whose arrangements determine physical and chemical behavior.
Example
A balloon filled with helium contains matter in the gas state, while the rubber balloon itself is matter in the solid state.
Common confusion
Matter is not the same as mass or energy. Mass is a property of matter, and energy can move matter around, but energy itself is not matter.