Simple Definition:

A solution is a homogeneous mixture [a mixture where all the parts are evenly distributed] of two or more substances. The substance present in a larger amount is called the solvent [the substance that dissolves the other substances], and the substance present in a smaller amount is called the solute [the substance that dissolves in the solvent]. Examples of solutions are saltwater (where the solvent is water and the solute is salt) and sugar water (where the solvent is water and the solute is sugar).

Very Simple Definition:

A solution is a mix of two or more things. For example, when you mix sugar in water, you get a solution.