Concentration
Concentration is how much substance sits in each millilitre of liquid, usually written as mg/mL.
It is a ratio, not a quantity, and it only exists once something has been dissolved. Divide the amount in the container by the volume of liquid added and you have it.
Concentration is the hinge every calculation on this site turns on. It converts an amount into a volume, and a volume into a mark on a syringe. Change the liquid volume and you change the concentration, without changing how much compound you have.
For example
10 mg of compound in 2 mL of liquid is 5 mg/mL. The same 10 mg in 1 mL is 10 mg/mL — twice as concentrated, same amount of compound.
What it is not
Concentration is not strength in the everyday sense and not a measure of quality or purity. A more concentrated solution is not a better one; it just has less liquid in it.
Where this comes up: reconstitution, reverse, syringe-units