'middle-income' originates from the combination of 'middle,' meaning 'intermediate or average,' and 'income,' referring to money received, especially on a regular basis, for work or through investments.

'Middle-income' evolved from the need to categorize economic classes, particularly in the 20th century, as societies became more economically diverse.

Initially, it referred to the average earnings of individuals, but over time it has come to represent a socio-economic class.