Mixture
A mixture is a combination of two or more substances that are physically mixed, not chemically bonded, and can be separated by physical means.
Definition
A mixture is two or more substances stirred together without bonding into something new; each part keeps its own properties and can be separated again. The components are pure substances combined physically in any proportion with no chemical bonds between them, and mixtures are classified as homogeneous (uniform, like salt water) or heterogeneous (visibly different parts, like sand in water). Their properties are typically intermediate between, or a weighted blend of, the components. By particle size, mixtures divide into solutions (particles below about $1\text{ nm}$), colloids ($1$ to $1000\text{ nm}$), and suspensions (larger particles that settle), and separation exploits differences in physical properties such as boiling point, solubility, density, or particle size through filtration, evaporation, distillation, magnetism, or chromatography.
Example
Trail mix is a mixture of nuts, raisins, and chocolate; you can pick out the raisins, and they are still raisins. Air is a mixture of about $78\%$ nitrogen, $21\%$ oxygen, and $1\%$ argon and other gases, and the ratio can change (humid air has more water vapor), which is one sign it is a mixture and not a compound. Crude oil is a mixture of hundreds of hydrocarbons, and fractional distillation separates them by boiling point: gases below $40^\circ\text{C}$, gasoline roughly $40$ to $200^\circ\text{C}$, kerosene and diesel above that, and heavy residues that never boil at atmospheric pressure.
Key Insight
Making a mixture is a physical change, not a chemical one, so nothing new is created and nothing has to be "un-made" to separate it. Iron and sulfur powder stirred together is a mixture and a magnet pulls the iron out; heat them and they react into iron sulfide, $FeS$, a compound the magnet ignores, the same atoms in a completely different situation. Colloids like milk and fog look homogeneous but scatter light (the Tyndall effect) because their particles are large enough to deflect a beam, so a laser pointer through a true solution is invisible but through milk it glows.