The total energy radiated by an object at all wavelengths, usually given in joules per second (identical to watts).
Luminosity is generally understood as a measurement of brightness. Each discipline, however, defines the term differently, depending on what is being measured.
Image of galaxy NGC 4945 showing the huge luminosity of the central few star clusters, suggesting there are 10 to 100 supergiant stars in each of these, packed into regions just a few parsecs across.
In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object in joules per second, which are watts. A watt is one unit of power, and so just as a light bulb is measured in watts, so too is the Sun, the latter having a total power output of 3.846×1026 W. It is this number which constitutes the basic metric used in astronomy and is known as 1 solar luminosity.Radiant power, however, is not the only way to conceptualize brightness, so other metrics are also used. The most common is apparent magnitude, which is the perceived brightness of an object from an observer on Earth at visible wavelengths.
Source: wikipedia, NASA