The difference is not completely clear-cut.
A reagent is a substance added to a system to cause a chemical reaction.
It is often dissolved in a solvent before it is added as a solution to the reaction mixture.
Chemists sometimes call a solution a reagent if it is used to test for the presence of another substance, for example, by a colour change or the formation of a precipitate.
For example, Tollens' reagent is a colourless, basic, aqueous solution containing diamminesilver ions
In the Tollens test for aldehydes, these ions react with aldehydes to form a silver mirror on the inside of the container.