BL Lacertae lies in the southern parts of the small northern constellation of Lacerta. Though a very faint object as seen from Earth, it is also highly variable, ranging in apparent brightness between about fourteenth and seventeenth magnitude. In the late 1920's this led to its being classified as a variable star, and designated using the standard variable naming conventions as 'BL Lacertae'.
It was not until some forty years later that this initial classification was found to be mistaken: BL Lacertae is not a star at all, but an immensely distant object, probably some 900 million light years beyond the Milky Way. At the heart of a nebulous, near-spherical Elliptical Galaxy is a dense and compact core. The black hole within that core causes an intense jet of matter to be emitted in nearly the line of sight of an observer on Earth, and oscillations in that jet are thought to be the source of its shifts in brightness (and also of intense bursts of radio activity).
Though the first object of this type to be identified, BL Lacertae is not unique, and active galaxies of this kind discovered subsequently are designated 'BL Lacertae Objects' (typically abbreviated to 'BL Lac Objects').
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