∞ generated and posted on 2016.08.18 ∞
The amount of something per unit volume.
In chemistry, concentration units are often in grams per ml or moles per liter (molarity), which are both examples of weight-volume concentrations. Weight-weight is also possible, such as g per 100g of overall material, which provides a percentage measure, etc. The term "density" often can be used synonymously to concentration.
Note that increasing volume but not changing number will lower concentrations while decreasing volume but not changing number will increase concentration (similarly, increasing number without changing volume will increase concentration while decreasing number but not changing volume will decrease concentration).