Fugacity is a technical term that describes the tendency of a substance to escape from a closed system. However, there are many synonyms for this word that can be used in everyday language. Some of these synonyms include volatility, evanescence, fleetingness, transience, impermanence, and ephemerality. These words all describe something that is temporary or fleeting in nature, and they can be used to describe things like emotions, thoughts, or even fleeting moments of beauty or joy. While the word fugacity may be more appropriate in scientific contexts, these synonyms can be useful for everyday conversation and writing.