A solvent is a substance that can dissolve other substances [break them down into smaller parts that mix evenly in the solvent]. Common examples include water, alcohol, and acetone [a liquid used for removing nailpolish]. For instance, when salt is mixed in water, the water is the solvent that dissolves the salt.
Very Simple Definition:
A solvent is something that can make other things disappear in it. Water is an example of a solvent because it can make salt disappear when mixed together.