Definition of “solution”
This is what Encyclopaedia Britannica definition of a solution in chemistry:


  Solution, in chemistry, a homogeneous mixture of two or more substances in relative amounts that can be varied continuously up to what is called the limit of solubility. The term solution is commonly applied to the liquid state of matter, but solutions of gases and solids are possible. Air, for example, is a solution consisting chiefly of oxygen and nitrogen with trace amounts of several other gases, and brass is a solution composed of copper and zinc.


A solution consists of solutes (at least one) and a solvent. The solute is define as the substance that is dissolved in the solvent. In other words, for solutions with components in the same phase, the substance(s) present in lower concentration are solutes while the major substance present in highest abundance is the solvent. For example, 190-proof alcohol is a liquid-liquid solution, which is the mixture of 95% ethanol and 5% water by volume. The major substance in this mixture is ethanol. Therefore, it is the solvent. The 5% water is the solute. Brass is a solid-solid solution of copper and zinc metals. General composition of brass is 65% copper and 35% zinc by weight. Hence, in brass, copper is the solvent and zinc is the solute. Air is a solution of multi-gases. In air, 72% nitrogen and 20% oxygen, and rest is other gases including $\ce{CO2}$ and $\ce{H2}$. Thus, the solvent of air is $\ce{N2}$ while $\ce{O2}$ is one of its solutes. Saline solution, on the other hand, is a solid-liquid solution. In a saline solution, solid salt ($\ce{NaCl}$) is the solute dissolved in water, which is the solvent (liquid).