The line doesn't seem to be overflowing with serials definitions, so
I would suggest that the obvious place to look is the glossary
at the back of AACR2r where a pretty good definition is given:

Serial. A publication in any medium issued in successive parts bearing
  numeric or chronological designations and intended to be continued
  indefinitely.  Serials include periodicals; newspapers; annuals
  (reports, yearbooks, etc.); the journals, memoirs, proceedings,
  transactions, etc., of societies; and numbered monographic series.

Hope this gets to you in time for your presentation tomorrow Susan!