Some of the properties of Optically Violent Variables (OVVs) are shared by ``BL Lac objects'', named after the prototype of the class, BL Lacertae, which was originally identified as a highly variable star, as one can conclude from its variable-star name. BL Lac objects are distinguished by the absence of strong emission or absorption lines in their spectra. Weak stellar absorption features or weak nebular emission lines often can be detected in very high signal-to-noise ratio spectra, however.

It is thought that both OVVs and BL Lacs are those AGNs which have a strong relativistically beamed component close to the line of sight. Collectively, OVVs and BL Lacs are sometimes referred to as ``blazars''. All known blazars are radio sources.

Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)