When something is described as "once-in-a-lifetime," it means that it is a unique experience that cannot be replicated. There are many synonyms for this phrase that can be used to describe special moments and events. For example, you could say that something is "rare," "unique," or "one-of-a-kind." Other synonyms include "exceptional," "extraordinary," "singular," and "unprecedented." When something is truly special, you might describe it as a "miracle," a "wonder," or a "dream come true." Whatever synonym you choose, it's clear that "once-in-a-lifetime" experiences are some of the most memorable and meaningful moments in our lives.