One-time is a phrase that refers to something that happens only once. It may be a singular event that occurs at a specific date and time, or a rare occurrence that happens only once in a lifetime. There are a variety of synonyms for the term "on one time", including "singular", "unique", "once-only", and "occasional". Each of these words carries its own connotation and nuance, and each can be used in different contexts. If you're looking for a way to express the idea of a one-time event or occurrence, try experimenting with these synonyms to find the one that best captures the essence of what you want to convey.