A country is described as developing if

A developing country is characterised primarily by a low income per head (low per capita income). Per capita income measures the average income earned per person in a country and is the most widely used indicator of a nation's level of economic development.

When income per head is low, it signals that the country's total output relative to its population is small, which typically correlates with limited industrialisation, lower standards of living, inadequate infrastructure, and reduced access to healthcare and education.

Low labour supply does not define a developing country. In fact, many developing countries have abundant labour, often with high rates of underemployment. A decreasing population is not a characteristic of developing nations either; most developing countries experience population growth rather than decline. A high contribution of the tertiary (services) sector to national income is actually a feature of developed economies, where the economy has progressed beyond primary and secondary production into services, finance, and technology.

The defining feature remains low per capita income, which reflects the overall economic productivity and living standards of the population.