BL Lacertae objects are radio galaxies
characterized by rapidly varying luminosity, no
emission lines in the spectrum, and point-like
appearance. As with blazars, they are aligned so
that their plasma jets point in our direction. (BL
Lacertae objects can be defined as a blazar whose
spectrum is dominated by the frequencies of (to us)
visible light.)
(The name is actually a remnant from an original
misidentification. The first observed BL Lacertae
galaxy was mistaken for a variable star in our own
galaxy that was named BL Lacertae. Only later was it
realized that it was not a variable star but a distant
galaxy exhibiting these peculiar properties.
Nevertheless, the name stuck to the class of objects
exhibiting the same features.)