The term "under-developed" refers to something that is not fully developed or lacks progress. However, there are several antonyms to it, which reflect a different perspective altogether. For instance, the term "developed" denotes progress, growth, and advancement in a particular field, whereas "matured" refers to reaching a level of completeness or perfection. Similarly, "established" suggests stability and a strong foundation, while "flourishing" hints at prosperity and success. Other antonyms for under-developed include "expanded," "evolved," "modernized," "progressive," "refined," "sophisticated," and "upgraded." Using these antonyms can bring a positive tone to a sentence or conversation, highlighting progress and success rather than lack thereof.