Astronomers define star brightness in terms of apparent magnitude — how bright the star appears from Earth — and absolute magnitude — how bright the star appears at a standard distance of 32.6 light-years, or 10 parsecs. (A light-year is the distance light travels in one year — about 6 trillion miles, or 10 trillion kilometres.) Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface.
My source: https://www.space.com/21640-star-luminosity-and-magnitude.html
