The concept of development can mean various things depending on what field you are referring to. Generally, development refers to improvement and growth. It involves moving from a primitive condition and transitioning through stages to a more advanced condition. In some instances, what constitutes development is subjective, because there is an implied value judgment in what is considered to be an improvement.
In terms of economics, development refers to a rise in the standard of living of a given country. Economic development involves low-income economies transforming into more sophisticated and prosperous ones. Economists study how a country's economic output can increase in terms of GDP and industrial and commercial advancements. This will usually result in changes in that nation's society.
In sociology, development is often related to economic development, but the concept varies somewhat. Overall, societal development is concerned with how social change occurs. This involves changes in family structure, religion, and governance. Societal development also includes issues of population growth, migration, and assimilation with other societies.
In the field of psychology, the concept of development involves studying how people mature throughout their lives. Mostly, this is related to cognitive, emotional, social, and intellectual abilities. People's abilities to think and control their thoughts and emotions increases drastically as they age. Exactly how the brain changes to allow these developments has been a major area of study in psychology for years.