An adsorption isotherm is a curve that describes how molecules adhere to a solid surface at a constant temperature. It illustrates the relationship between the amount of adsorbate (the substance being adsorbed) and its concentration in the surrounding phase, typically a gas or liquid. This relationship helps in understanding how different materials, like activated carbon or zeolites, can capture and hold substances. There are several models of adsorption isotherms, including the Langmuir and Freundlich isotherms. Each model provides insights into the adsorption process, such as whether it occurs on a homogenous or heterogeneous surface. These isotherms are crucial in fields like environmental science and chemical engineering for designing effective adsorption systems.