Re: May i know what is fugacity in thermodynamics?

I assume you are asking for a physical understanding of the fugacity. If you just want the equation defining the fugacity, related to the chemical potential and so forth, you should be able to find that in any college textbook on physical chemistry, chemical thermodynamics, or chemical engineering thermodynamics.

One way to think of the fugacity is as an "escaping tendency." In fact, it comes from the same word as "fugitive." The fugacity measures how much a substance wants to "escape" from the phase it is in. Therefore, if two phases are in contact and the fugacity of a component is higher in one phase, that component will migrate to the other phase until, at equilibrium, the fugacities are equal (one of the conditions for phase equilibria is that the fugacity of any given component is the same in phase A as it is in phase B). You can also think of this as a measure of "discomfort" -- the more "uncomfortable" a molecule is in a given phase, the higher its fugacity and the more it will want to escape into another phase where its fugacity is lower. Or, in the context of reaction thermodynamics, equilibrium constants are (or should be) written in terms of fugacities, and raising the fugacity of a component will make it tend to "escape" by pushing the reaction equilibrium in the direction that consumes that component.

It is also sometimes useful to think of the fugacity as a "corrected partial pressure." Recall that the partial pressure is the mole fraction of a component times the total pressure. For an ideal gas, the fugacity is identical to the partial pressure (a quantity called the fugacity coefficient is the ratio of the fugacity to the partial pressure; it is one for an ideal gas). So you can think of the fugacity as a correction to the partial pressure, accounting for the fact that real substances are not ideal gases.