In general, AGNs show continuum variability at all wavelengths at
which they have been observed, from X-rays to radio wavelengths. A
small subset of AGNs show short time-scale variations that are
abnormally large, e.g. m
0.1 mag in the visible
wavelengths on time
scales as short as a day. In addition to their large variations in
flux, they also tend to have high polarization (up to a few percent,
as opposed to less than 1% for most AGNs) which also varies, in both
magnitude and position angle, and these sources are always radio
loud. These AGNs are known as ``optically violent variables'', or OVVs.
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)