Indigo is a flowering plant that's native to Asia, and it's also the name of a deep blue dye made from the leaves of the indigo plant. You can also use the word to mean the shade of blue itself.
The indigo plant is in the pea family. While this plant often has brightly colored flowers, indigo dye is traditionally made from the plant's leaves. This type of tint was once the most common natural dye in the world — though today, most of what's called indigo is synthetically made. Indigo is also one of the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. The Greek root indikon means "blue dye from India."