concentration

Concentration is the amount of solute present in a fixed quantity of a solution or solvent.

• A solution with relatively little solute is called dilute, and one with a lot is concentrated — but these are relative terms, not fixed thresholds, so "concentrated" only makes sense compared to something else.
• Dissolving one spoon of salt in water gives a more dilute solution than dissolving two or more spoons in the same amount of water — concentration tracks how much solute is packed into a given amount of liquid.
• Concentration and solubility are closely related but not the same thing: solubility is the maximum possible concentration a solvent can hold at a given temperature, while concentration is simply how much solute happens to be dissolved right now.
• Comparing concentrations sometimes takes care — 2 spoons of salt in 100 mL of water and 4 spoons in 50 mL aren't obviously comparable at a glance, since both the amount of solute and the amount of solvent differ.