Developed describes something that has grown, improved, or become more advanced over time through effort and progress. A developed skill is one you've practiced and refined: a pianist who can play complex pieces has developed her abilities through years of practice. A developed idea is one that's been thought through carefully, with details and evidence added to make it stronger and more complete.

Countries are often called developed or developing based on their infrastructure, economy, and technology. Developed countries typically have extensive road systems, reliable electricity and water, advanced healthcare, and widespread education. Think of the difference between a rough trail through the woods and a paved highway with signs, lights, and rest stops: that transformation from basic to sophisticated represents development.

In photography, to develop film means to treat it with chemicals so the images appear. This older meaning captures something important about the word: development is about making something visible or real that was only potential before. When you develop an argument in an essay, you're taking a basic claim and building it out with examples and reasoning until readers can clearly see your point. Development transforms the simple into the sophisticated and the rough into the refined.