Compound
A compound is a pure substance made of two or more elements chemically bonded in a fixed ratio, with properties unlike its elements.
Definition
A compound is a substance made when two or more different elements bond together, and the result can look and act completely unlike the elements it came from. It is a pure substance whose atoms combine chemically in a fixed, definite ratio, represented by a chemical formula, and it can only be broken back into its elements by a chemical change. Compounds are either molecular, held together by covalent bonds into discrete molecules, or ionic, held by attractions between ions in an extended lattice that is described by an empirical formula unit. The fixed stoichiometric proportions are expressed by the law of definite proportions, and a compound's properties depend on its bonding and structure rather than on the properties of its free elements.
Example
Sodium is a soft metal that explodes in water and chlorine is a poisonous green gas, yet bonded together they make sodium chloride, the table salt you sprinkle on food. Water is always $H_2O$, two hydrogen atoms for every oxygen atom by count and about $11\%$ hydrogen by mass, while hydrogen peroxide, $H_2O_2$, has a different ratio and is a different compound with different properties. Sodium chloride has a molar mass of $58.44\text{ g/mol}$ and a face-centered cubic lattice in which each $Na^+$ is surrounded by six $Cl^-$; there is no discrete $NaCl$ molecule, so $NaCl$ is a formula unit, and its lattice energy of about $787\text{ kJ/mol}$ explains its melting point of $801^\circ\text{C}$.
Key Insight
A compound is not a mixture: you cannot separate salt back into sodium and chlorine by filtering or picking it apart, it takes a chemical reaction. The fixed ratio is the key, since carbon monoxide ($CO$) is a deadly gas and carbon dioxide ($CO_2$) is what you exhale, and one extra oxygen atom per molecule makes them different compounds. Proust's law of definite proportions was evidence that atoms exist, because only whole atoms combining in whole-number ratios can explain why a compound always has the same composition regardless of where it was found or how it was made.