solution
A solution is what you get when sugar disappears into your tea: the solid does not just sit at the bottom, it breaks apart into individual molecules that spread evenly through the liquid. Look as hard as you like and you will see no particles, no cloudiness, no separate layer — just one clear, uniform liquid.
More precisely, a solution is a homogeneous mixture in which a solute is dispersed at the molecular or ionic level throughout a solvent. Because the dispersed units are smaller than the wavelength of light, true solutions do not scatter light and stay clear. The composition is uniform everywhere, so a sample taken from the top has the same concentration as one taken from the bottom.
In pharmacy, solutions are valued because the drug is already dissolved and so does not have to dissolve in the body before it can be absorbed — this often gives a faster and more reproducible response than a tablet. Oral syrups, elixirs, injections and eye drops are all solutions. The trade-off is that many drugs are poorly water-soluble, so getting and keeping them in solution can be the hardest part of formulation, and a dissolved drug may also be chemically less stable than a dry solid.
Normal saline for injection is a solution of 0.9% w/v sodium chloride in water for injection — the salt is fully dissociated into sodium and chloride ions, leaving a perfectly clear, colourless liquid.
A clinical solution where the solute is fully dissolved and invisible.
Do not confuse a solution with a suspension or colloid. A suspension contains visible solid particles that settle and can be filtered out; a colloid scatters light (the Tyndall effect). A true solution does neither — it is clear and the solute cannot be removed by ordinary filtration.