The amount of one substance in a certain volume of another is called concentration.

Here's a breakdown:

* Concentration refers to the amount of a solute (the substance being dissolved) present in a given amount of solvent (the substance doing the dissolving).

* Volume is the amount of space a substance occupies.

Therefore, concentration expresses the ratio of the solute to the solvent, usually in terms of:

* Molarity (M): Moles of solute per liter of solution

* Molality (m): Moles of solute per kilogram of solvent

* Percent by mass (% w/w): Grams of solute per 100 grams of solution

* Percent by volume (% v/v): Milliliters of solute per 100 milliliters of solution

* Parts per million (ppm) or parts per billion (ppb): Units of solute per million or billion units of solution, respectively.

Let me know if you'd like a specific example or want to explore different units of concentration!