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]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "newsletter":
   Pastoral Epistle, advice, aerogram, air letter, airgraph,
   billet-doux, broadcast journalism, bull, chain letter, dead letter,
   dimissorial, dimissory letter, drop letter, encyclical, fan letter,
   form letter, information, intelligence, journalism,
   letter of credit, letter of introduction, letters credential,
   letters of marque, letters of request, letters overt,
   letters patent, letters rogatory, love letter, monitory, news,
   news agency, news medium, news service, newsiness, newsmagazine,
   newspaper, newsworthiness, nixie, open letter, pastoral letter,
   poison-pen letter, press association, radio, reportage,
   round robin, telegraph agency, television, the fourth estate,
   the press, tidings, wire service, word



The Free On-line Dictionary of Computing (30 December 2018):

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)