As physical adsorption is reversible, it is accompanied by decrease in pressure. Therefore, it is expected that at a given temperature the extent of adsorption will increase with the increase of pressure of the gas.
The extent of adsorption is measured as x/m where m is the mass of adsorbent and x that of adsorbate.
If the physical adsorption is limited to unimolecular layer, the plot of x/m vs. equilibrium pressure at a constant temperature is as shown.

It is evident from the graph that at a certain pressure the adsorption reaches a maximum value i.e. the adsorption becomes saturated and the corresponding pressure is called saturation pressure (Ps). Beyond this pressure the adsorption remains constant.

The plot between the extent of adsorption (x/m) against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

Freundlich adsorption isotherm:

Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.