Adsorption isotherm:
The curve showing the variation of adsorption with pressure at constant temperature is known as the adsorption isotherm. It represents the relationship between the amount of adsorbate (substance being adsorbed) and the pressure at a constant temperature.

Explanation:
Adsorption is the process in which molecules of a substance adhere to the surface of a solid or liquid, forming a thin film. The adsorption isotherm describes how the amount of adsorbate molecules on the surface of the adsorbent changes with pressure. It is typically represented graphically as a curve.

Key Points:
- Adsorption isotherms are obtained by conducting experiments where the pressure of the adsorbate is varied while keeping the temperature constant.
- The adsorption isotherm curve can have different shapes depending on the type of adsorption, nature of adsorbate, and adsorbent.
- The most common type of adsorption isotherm is the Langmuir isotherm, which assumes a monolayer coverage of adsorbate molecules on the surface of the adsorbent.
- Other types of adsorption isotherms include the Freundlich isotherm, BET isotherm, and Temkin isotherm, which are used to describe specific adsorption processes.
- The adsorption isotherm provides important information about the adsorption capacity, adsorption kinetics, and surface properties of the adsorbent.
- It is widely used in various industries, such as wastewater treatment, gas separation, and catalysis, to understand and optimize adsorption processes.

Conclusion:
The curve showing the variation of adsorption with pressure at constant temperature is known as the adsorption isotherm. It provides valuable insights into the adsorption process and is widely used in various applications.