A source that emits only one frequency of visible light is referred to as a monochromatic source. The term "monochromatic" comes from the Greek words "mono" meaning "single" and "chroma" meaning "color." In the context of light, it means that the source emits light of a single color or wavelength.
In practical terms, a source that emits light at only one frequency in the visible spectrum would produce light of a specific color. For example, if the source emits light with a wavelength of around 600 nanometers, it would appear as red light. Similarly, a source emitting light with a wavelength around 500 nanometers would produce green light, and so on.
It's important to note that in reality, sources like the Sun emit a broad range of frequencies or colors of visible light, resulting in white light. However, specialized light sources, such as lasers, can be engineered to emit light at a specific wavelength, making them effectively monochromatic sources for practical purposes.