The first recorded use of indigo as a color name in English was in 1289. Though the word indigo has existed in English since the 13th century, it may never have been a common part of the basic color-naming system.

Indigo is a deep and bright shade of blue. The color is named after the blue dye derived from the plant Indigofera tinctoria and related species. The color is placed on the electromagnetic spectrum between about 420 and 450 nm in wavelength, placing it between blue and violet. Although traditionally considered one of seven colors of the rainbow or the optical spectrum, modern color scientists do not usually recognize indigo as a separate division and generally classify wavelengths shorter than about 450 nm as violet.

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo dye, derived from the plant Indigofera tinctoria, to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the 'dye,' which was indikon. The Romans used the term indicum, which passed into Italian dialect and eventually into English as the word indigo.