Solvent
A solvent is the substance that does the dissolving in a solution, usually the component present in the larger amount; water is the most common solvent.
Definition
The solvent is the substance that does the dissolving, usually the bigger amount and often a liquid. It is the component of a solution present in the greatest amount, into which the solute dissolves; solvents are usually liquids, but the solvent in a solid solution like brass is the major metal, and nitrogen is the solvent in air. Polar solvents such as water dissolve polar and ionic substances, while nonpolar solvents such as oil dissolve nonpolar substances, because a solvent's ability to dissolve a solute depends on the balance of intermolecular forces: hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Solvent polarity, dielectric constant, and hydrogen-bonding capacity determine which solutes dissolve and how fast reactions proceed in solution, and water's high dielectric constant (about $80$ at $20^\circ\text{C}$) weakens the attraction between ions, which is why it dissolves ionic compounds so well.
Example
In hot chocolate, the milk is the solvent and the cocoa powder is the solute, and water is the solvent in almost every drink. Nail polish will not come off with water but dissolves quickly in acetone, a solvent that matches the nonpolar polish better than water does. Hexane ($C_6H_{14}$) is nonpolar and dissolves fats and waxes through dispersion forces but will not dissolve $NaCl$, because it cannot stabilize separated ions, while ethanol, with a polar $OH$ group and a nonpolar ethyl group, dissolves both water and many organic compounds, making it a bridge solvent.
Key Insight
Water is called the "universal solvent" because it dissolves more things than any other common liquid, which is why perfectly pure water is so hard to find in nature. Water dissolves salt and sugar but not oil, and oil dissolves grease but not salt, so "like dissolves like" is the rule for picking a solvent. Green chemistry pushes toward water and supercritical $CO_2$ as solvents because many traditional organic solvents are toxic or flammable, and choosing the solvent is often the biggest environmental decision in an industrial process.