solution
/ suh-LOO-shun /
Stir a spoon of sugar into tea and watch the white grains vanish. The sugar is still there — your tongue confirms it — but you cannot see it, filter it out, or let it settle. What you have made is a solution: a mixture so thoroughly blended that the pieces mingle one molecule at a time and look perfectly uniform everywhere you sample it.
More precisely, a solution is a homogeneous mixture of two or more substances. The component present in larger amount (usually the liquid you started with) is the solvent; whatever is dissolved into it is the solute. Solutions are not only liquids: air is a gas solution, and brass is a solid solution of zinc in copper. The defining feature is that the mixing happens at the scale of molecules and ions, so no boundaries or grains remain to be seen.
This matters because most chemistry — in test tubes, in the ocean, and inside your cells — happens in solution. Dissolving spreads molecules out and lets them meet and react freely. But "dissolved" is not the same as "reacted": a solution can usually be unmixed again, for example by boiling off the water and recovering the salt, because the components keep their own chemical identities.
Seawater is a solution: salts and minerals dissolved in water. You cannot see the salt, but boil the water away and a white crust of salt is left behind.
A solution looks uniform, yet its parts keep their own identities and can be recovered.
A solution is clear and stays mixed; a suspension (like muddy water) has particles big enough to settle, and a colloid (like milk) has particles in between that scatter light but stay afloat. Only the molecule-scale mixing of a true solution counts here.