Solution
A solution is a homogeneous mixture in which one substance, the solute, is dissolved evenly in another, the solvent, such as sugar in water.
Definition
A solution is what you get when one substance dissolves completely into another, spreading out so evenly that you cannot see it anymore. It is a homogeneous mixture of a solute dissolved in a solvent, where the solute is present in the smaller amount and the solvent in the larger; water is the most common solvent, solutions in water are called aqueous, and solutions can be solid, liquid, or gas with concentrations up to the solubility limit. At the particle level a solution is a single-phase mixture in which solute molecules or ions are dispersed at the molecular level throughout the solvent. Dissolution occurs when solute-solvent attractions are comparable to or stronger than the solute-solute and solvent-solvent attractions being broken, summarized as "like dissolves like," and dissolved solutes lower the freezing point and raise the boiling point of the solvent (colligative properties).
Example
Stir salt into water and the salt seems to vanish, but it is still there (you can taste it), so the salty water is a solution. Dissolving $10\text{ g}$ of sugar in $100\text{ g}$ of water gives $110\text{ g}$ of solution, with the sugar as solute, the water as solvent, and a mixture about $9\%$ sugar by mass. When $NaCl$ dissolves, the polar water molecules orient their partially negative oxygen atoms toward $Na^+$ and their partially positive hydrogens toward $Cl^-$, and the hydration energy released offsets the $787\text{ kJ/mol}$ lattice energy; a $0.50\text{ m}$ $NaCl$ solution freezes at about $-1.9^\circ\text{C}$ because it releases two moles of particles per mole of salt.
Key Insight
Dissolving is not disappearing: the salt breaks into pieces too small to see, but the water gets heavier by exactly the mass of the salt. Solutions are transparent (though they can be colored) because the dissolved particles are too small to scatter light, so cloudy water is a suspension, not a solution. Blood plasma, seawater, and the electrolyte in a battery are all solutions whose chemistry depends on concentration, and road salt melts ice by freezing-point depression, the same colligative effect that lets antifreeze protect an engine.