Simplified (very) definition of serial:

Any publication that has successive parts but no planned ending. For
example: the Corpus Inscriptionum Latinorum, however humongous a set it
ends up being, is not a serial -- someday, the very last extant Latin
inscription will be found and published.  Newspapers, journals, bulletins,
magazines, annual reports, and the like ARE serials, published on the
best-of-all-possible-worlds assumption that they will continue ad
infinitum.

Some libraries make a distinction between serials and periodicals:
"periodical" is then often the term used for a serial with articles,
e.g. newspapers, journals, magazines.