Lights are indeed a rainbow of colors. Sunlight is made up of seven different colors. Monochromatic means 'of the same color.'
Technically it just has a specific wavelength, no practical electromagnetic radiation is actually entirely monochromatic, hence monochromatic light is generally stated to get a wavelength within such a limited wavelength range.
Source of monochromatic light:
LASER stands for Light amplification by stimulated emission of radiation. It is a popular monochromatic light source.
Laser light is produced by a singular or unique atomic transition with such a single wavelength, resulting in what seems like a monochromatic beam of light.
Therefore, monochromatic light has been defined as almost any light with a single wavelength.