Solute
A solute is the substance that gets dissolved in a solution, usually the component present in the smaller amount, such as salt in salt water.
Definition
The solute is the substance that dissolves, usually the smaller amount, the thing you stir in. It is the component of a solution dissolved in the solvent, and it can be a solid, liquid, or gas; a solution can hold several solutes at once, and when a solute dissolves its particles separate and become surrounded by solvent particles. Ionic solutes dissociate into hydrated ions and conduct electricity (electrolytes), while most molecular solutes remain intact and do not (nonelectrolytes). The amount of solute is quantified by concentration units such as molarity (moles of solute per liter of solution) and molality (moles of solute per kilogram of solvent).
Example
When you make lemonade, the sugar you stir in is the solute and the water is what it dissolves into. Seawater contains about $35\text{ g}$ of dissolved solids per kilogram, mostly sodium chloride along with magnesium, sulfate, and calcium ions, and each of those dissolved substances is a solute in the water solvent. Dissolving $0.10\text{ mol}$ of $CaCl_2$ in $1.0\text{ kg}$ of water produces $0.10\text{ mol}$ $Ca^{2+}$ and $0.20\text{ mol}$ $Cl^-$, or $0.30\text{ mol}$ of dissolved particles, so its freezing-point depression is about three times that of $0.10\text{ mol}$ of sugar, a nonelectrolyte, in the same water.
Key Insight
The solute does not have to be a solid; carbon dioxide gas is the solute in soda water. Solutes change the properties of the solvent, so adding salt to water raises its boiling point and lowers its freezing point, and the more solute you add, the bigger the shift. Colligative properties count particles, not identity, which is why a van't Hoff factor is needed for ionic solutes and why measuring freezing-point depression was once a standard way to determine whether a solute dissociates in solution.