Fugacity is the quality of impermanence. With plants, fugacity refers to the parts that drop off. In chemistry, it’s the tendency of a gas to expand till it dissipates. Fugacity can also refer to things that don’t last, like youth.
Anything fugacious is prone to run away or escape. The noun form, fugacity, has two related meanings. The first is the quality possessed by gases that expand and eventually dissipate. Some gases have more fugacity than others. Similarly, fugacity refers to changeableness in general. Anything that is prone to change, erode, or die possesses fugacity. If something is eternal and unchanging, it lacks fugacity.