活动星系核（Active Galactic Nucleus，缩写为AGN）是一个星系中央区有比普通的星系的强烈很多的光度，至少部分波段或甚至可能全部波段里都发出很强的电磁波谱。被观察到的发射复盖从无线电波，微波，红外线，可见光，紫外线，X射线，到伽玛射线。光度大约在10-10J/s之间。容纳活动星系核的宿主星系为活跃星系。活跃星系核是这些星系明亮的核心部分，尺度通常在1光年上下，只占整个活跃星系的很小一部分。活动星系核（AGN）是在宇宙中的电磁辐射的最明亮的持久性的来源，并且因此可以被用作发现远方天体的方法；其演化的宇宙时间函数也设置了宇宙模型的制约条件。

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion – and possibly all – of the electromagnetic spectrum. Such excess emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from an AGN is believed to be a result of accretion of mass by a supermassive black hole at the center of its host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos.