Well-understood means fully comprehended or clearly grasped. There are several synonyms for this phrase that can be used interchangeably, depending on the context. For example, some alternatives to well-understood include well-known, familiar, well-recognized, or well-established. Additionally, commonly used synonyms for this phrase include well-versed, knowledgeable, well-informed, or well-acquainted. These phrases all convey a level of comprehension and understanding that is thorough and complete. Using a synonym for well-understood can help to add variety and nuance to your writing, while still conveying the same basic idea of fully comprehending a particular concept or idea.