At the University of Illinois in Urbana we use the following definition
of a serial:

A serial is a publication issued in successive parts at regular or
irregular intervals and intended to continue indefinitely.  Included
are periodicals, newspapers, proceedings, reports, memoirs, annuals
and numbered monographic series.  A periodical is a serial which is
usually published three or more times a year.  A continuation is a
serial which is usually published less than three times a year.

Betsy Kruger, Univ Illinois at Urbana-Champaign