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  逸度定義的出發點是化學勢與理想氣體的壓強的關係。  根據熱力學基本方程: formula_4 定溫下對壓強從參考態壓強(1 巴)開始作定積分: formula_5 對於理想氣體有 formula_6  整理後,得:  formula_7  上式給出了理想氣體等溫過程中化學勢與壓強的關係。  對於實際氣體,因為上述狀態方程式計算複雜,上述定積分難於處理計算,因此引入逸度的概念,逸度為修正因子,從而將實際氣體的化學勢表示式與理想氣體統一起來。 formula_8  一般把上式作為逸度的定義式,也可以明確地將formula_1解出: formula_10

  Fugacity is used to better approximate the chemical potential of real gases than estimations made using the ideal gas law. Yet fugacity allows the use of many of the relationships developed for an idealized system.  In the real world, gases approach ideal gas behavior at low pressures and high temperatures; under such conditions the value of fugacity approaches the value of pressure. Yet no substance is truly ideal. At moderate pressures real gases have attractive interactions and at high pressures intermolecular repulsions become important. Both interactions result in a deviation from "ideal" behavior for which interactions between gas atoms or molecules are ignored.  For a given temperature formula_11, the fugacity formula_12 satisfies the following differential relation:
逸度(英語:Fugacity)在化學熱力學中表示實際氣體的有效壓力,用 formula_1 表示。它等於相同條件下具有相同化學勢的理想氣體的壓強。在與化學勢有關的描述理想氣體性質的熱力學式子中用逸度代替活度,即可得到相應的描述實際氣體性質的關係式。例如 0°C 下 100個大氣壓的氮氣的逸度為 97.03 個大氣壓,這意味著它與 97.03 個大氣壓下的氮氣理想氣體有著相同的化學勢。逸度可以通過實驗測定,也可以用範德華氣體模型估算。逸度與壓強的比值稱為逸度係數formula_2,它是無量綱量。如下式所示。