Once is a common adverb that refers to a particular moment in time, usually in the past. However, there are also many other words and phrases that can convey a similar meaning. For example, you can use words like formerly, previously, formerly, or in the past to replace once. Also, you can use phrases like at one time, in earlier times, in days gone by, or long ago to replace it. Alternatives for once can also include terms like once upon a time, in a past era, one time, or in a bygone era. Substituting synonyms for once can enrich your writing, making it more colorful and varied.