BL Lacertae or BL Lac is a highly variable, extragalactic active galactic nucleus (AGN or active galaxy). It was first discovered by Cuno Hoffmeister in 1929, but was originally thought to be an irregular variable star in the Milky Way galaxy and so was given a variable star designation. In 1968, the "star" was identified by John Schmitt at the David Dunlap Observatory as a bright, variable radio source. A faint trace of a host galaxy was also found. In 1974, Oke and Gunn measured the redshift of BL Lacertae as z = 0.07, corresponding to a recession velocity of 21,000 km/s with respect to the Milky Way. The redshift figure implies that the object lies at a distance of 900 million light years.
thumb|left|A [[Photometric_system#Photometric_letters|visual band light curve for BL Lacertae, plotted from AAVSO data
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