concentration
/ kon-sen-TRAY-shun /
Think of a glass of squash. A little syrup in lots of water tastes weak; the same amount in half the water tastes strong. Concentration is exactly that idea made precise: how much of a substance is packed into a given amount of mixture.
Formally, concentration tells you the quantity of one component (the solute, the thing dissolved) relative to the amount of solution or solvent. The 'quantity' can be measured many ways — number of moles, mass, or volume — and each choice gives a different concentration unit, such as molarity, molality, or parts per million.
Concentration matters because almost every chemical measurement and recipe depends on it: a drug dose, a pollutant limit, the strength of an acid. Always state the units, because the same number can mean wildly different things depending on whether you counted moles or grams, per litre or per kilogram.
A label reading '9 g of salt per litre of solution' describes the concentration of saline; the same fluid could also be written as 0.15 mol/L or 0.9% by weight, depending on the unit chosen.
One physical solution, three valid concentration statements.
'Concentrated' and 'dilute' are only relative words — they tell you nothing numerical. Always pin a concentration to a unit before you use it in a calculation.