The Collaborative International Dictionary of English v.0.48:

newsletter \news"let`ter\, news-letter \news"-let`ter\, n.
   A circular letter, written or printed for the purpose of
   disseminating news. This was the name given to the earliest
   English newspapers.

   Syn: newssheet.
        [1913 Webster]

WordNet (r) 3.0 (2006):

newsletter
    n 1: report or open letter giving informal or confidential news
         of interest to a special group [syn: newsletter,
         newssheet]

The Free On-line Dictionary of Computing (19 January 2023):

newsletter

    A periodically published work containing news
   and announcements on some subject, typically with a small
   circulation.  Newsletters are a common application for DTP
   and may be distributed by electronic mail.

   (1996-12-10)