A substance is soluble if it dissolves in a fluid. The fluid (present in excess) is called the solvent and the substance dissolved in it is called the solute which together form a solution. The process of dissolving is called solvation. A solution that can not hold any more solute is said to be saturated.
The degree one substance dissolves in another is determined by its electrostatic attraction for the solvent and also the entropy change that accompanies the solvation. Some substances are soluble in alcohol which are not soluble in water.
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 compund, will dissolve in water, but not in ethanol.