Lights are a combination of colours. Sunlight is a combination of seven colours. Monochromatic means “same colour”. Basically monochromatic is any light which is purely of a single colour. Technically it has only a single wavelength, though, no real electromagnetic radiation is purely monochromatic, therefore monochromatic light is said to have a wavelength within a very short wavelength range.
Monochromatic source = a light source emitting one colour of light (flash units are monochromatic, usually 5000K). Simply put, the light which has the same wavelength will show only a single colour and this light will be monochromatic.