Well established is a common phrase that describes something that has been in existence for a long time and has a solid reputation. If you're looking to mix things up in your writing, consider using synonyms for this term. Examples include "firmly established," "long-standing," "recognized," "reputable," "settled," "proven," "entrenched," "steadfast," or "established history." Each of these phrases conveys the idea of something that has been around for a while and has gained a certain level of credibility. Choosing a synonym can add depth to your writing and keep your prose from becoming repetitive.