Matter
Matter is anything that has mass and takes up space, from air and water to rocks and living things; it is made of atoms.
Definition
Matter is anything that has mass and takes up space: your desk, the water in your bottle, and even the air you breathe. All matter is built from atoms, and it can be sorted by composition into pure substances (elements and compounds) and mixtures, and by state into solid, liquid, gas, and plasma. At the particle level, ordinary matter consists of protons, neutrons, and electrons, and the bulk properties you can see and measure emerge from how those particles are arranged, how fast they move, and what forces act between them. Light, sound, and heat carry energy but have no rest mass and occupy no volume, so they are not matter.
Example
A balloon full of air weighs slightly more than an empty balloon, which proves air is matter even though it is invisible. A $250\text{ mL}$ glass of water has a mass of about $250\text{ g}$; freeze it and the same $250\text{ g}$ is still there as ice. One mole of water, only $18.02\text{ g}$, contains $6.022 \times 10^{23}$ molecules, each made of two hydrogen atoms and one oxygen atom, so a small cup holds roughly $1.8 \times 10^{24}$ atoms.
Key Insight
Changing the state, shape, or mixing of matter never creates or destroys it, which is the idea behind the law of conservation of mass. Mass and energy are linked by $E = mc^2$, so in a nuclear reaction a tiny amount of matter can become an enormous amount of energy, but in ordinary chemical reactions the mass change is far too small to measure, and chemists treat mass as strictly conserved.