Monochromatic luminosity can be thought of as an astronomical object's total power output at a specific wavelength or frequency. (An interval of the spectrum of zero width will have zero power, but integrating the monochromatic luminosity over a finite interval reveals the finite power over that interval.) The power output over all wavelengths is termed the bolometric luminosity and is often what is meant by (plain) luminosity.
It is of note that an object's monochromatic luminosity per wavelength and that per frequency differ at the same point on the object's spectrum, the same curiosity as occurs with specific intensity. The relation between them is:
Lf dLf = Lw dLw