U3 is a term that can refer to various concepts, but it is often associated with a specific type of USB flash drive that includes a built-in software platform. This platform allows users to run applications directly from the USB drive without needing to install them on a computer. This feature makes U3 drives convenient for portable software solutions. Additionally, U3 can also refer to a classification in the United Nations system, specifically related to the Human Development Index. This classification helps assess the development levels of countries based on various socio-economic indicators, providing insights into global disparities.