(Verity)
``Topic agents'' are programs that filter on-line of off-line documents relevant to a particular topic. In that sense ``Query-by-theme'' and ``concept searching'' are particular cases of ``topic agents''. ``Topic agents'' may be used to gather off-line information of interest and warn a user of the existence of such information, for instance by sending him email.
See also ``query expansion'' and ``relevance ranking''.