A substance is soluble in a fluid if it dissolves in the fluid. The dissolved substance is called the solute and the fluid (present in excess) is called the solvent, which together form a solution. The process of dissolving is called solvation, or hydration if the solvent is water.
A solution at equilibrium cannot hold any more solute and is said to be saturated. Solutions may, under special conditions, hold more solute than the solvent can normally dissolve. This is called supersaturation.
The degree to which one substance dissolves in another is determined by the intermolecular forces between the solvent and solute, temperature, and the entropy change that accompanies the solvation.
Solvents are normally characterized as polar or nonpolar. Polar solvents will dissolve ionic compounds and covalent compounds which ionize, while nonpolar solvents will dissolve nonpolar covalent compounds. For example, ordinary table salt, an ionic compound, will dissolve in water, but not in ethanol.
Common solvents used in organic chemistry include acetone, ethanol, water, and benzene.
Water and nonpolar solvents are immiscible; they do not form homogenous mixtures but separate into two distinct phases or form milky emulsions.
While solutions are typically thought of as solids being mixed into liquids, any two states of matter can be mixed and be called a solution. Carbonated water is a solution of a gas in a liquid, and stainless steel is a solution of a solid in a solid (called an alloy).