Esprit d'escalier is a French term that describes the feeling of thinking of the perfect response or comeback after a conversation has ended. While there is no one-word synonym for this phrase, there are several phrases that convey the same sentiment. For instance, the term 'afterthought' is a word that describes an idea or response that comes to you after the moment has passed, similar to the feeling of esprit d'escalier. Other synonyms include 'retrospective wit,' 'staircase wit,' or 'epiphany after leaving.' All these phrases describe the same sense of regret felt when a witty or intelligent response is thought of after the conversation is over.