Presentation Open Access
The term "blazar" refers to a cosmic source with a relativistic jet viewed at a small angle, so that the electromagnetic emission is affected by beaming. Historically, blazars are divided into flat-spectrum radio quasars (FSRQs) and BL Lac Objects (BL Lacs). However, the recent discovery that also Seyfert galaxies (particularly Narrow-Line Seyfert 1 Galaxies, NLS1s) could host powerful relativistic jets, has set some problem of classification of jetted active galactic nuclei (AGN). I would like to review these problems and to advance one possible solution.