The Collaborative International Dictionary of English v.0.48:

Bulletin \Bul"le*tin\, n. [F. bulletin, fr. It. bullettino, dim.
   of bulletta, dim. of bulla, bolla, an edict of the pope, from
   L. bulla bubble. See Bull an edict.]
   1. A brief statement of facts respecting some passing event,
      as military operations or the health of some distinguished
      personage, issued by authority for the information of the
      public.
      [1913 Webster]

   2. Any public notice or announcement, especially of news
      recently received.
      [1913 Webster]

   3. A periodical publication, especially one containing the
      proceeding of a society.
      [1913 Webster]

   bulletin board, a board on which announcements are put,
      particularly at newsrooms, newspaper offices, etc.
      [1913 Webster] bullet-proof

WordNet (r) 3.0 (2006):

bulletin board
    n 1: a computer that is running software that allows users to
         leave messages and access information of general interest
         [syn: bulletin board system, bulletin board,
         electronic bulletin board, bbs]
    2: a board that hangs on a wall; displays announcements [syn:
       bulletin board, notice board]

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

bulletin board system
bboard
BBS
bulletin board
CBBS
message board

    (BBS, bboard /bee'bord/, message
   board, forum; plural: BBSes) A computer and associated
   software which typically provides an electronic message
   database where people can log in and leave messages.  Messages
   are typically split into topic groups similar to the
   newsgroups on Usenet (which is like a distributed BBS).
   Any user may submit or read any message in these public areas.

   The term comes from physical pieces of board on which people
   can pin messages written on paper for general consumption - a
   "physical bulletin board".  Ward Christensen, the programmer
   and operator of the first BBS (on-line 1978-02-16) called it a
   CBBS for "computer bulletin board system".  Since the rise of
   the World-Wide Web, the term has become antiquated, though
   the concept is more popular than ever, with many websites
   featuring discussion areas where users can post messages for
   public consumption.

   Apart from public message areas, some BBSes provided archives
   of files, personal electronic mail and other services of
   interest to the system operator (sysop).

   Thousands of BBSes around the world were run from amateurs'
   homes on MS-DOS boxes with a single modem line each.
   Although BBSes were traditionally the domain of hobbyists,
   many connected directly to the Internet (accessed via
   telnet), others were operated by government, educational,
   and research institutions.

   Fans of Usenet or the big commercial time-sharing bboards
   such as CompuServe, CIX and GEnie tended to consider
   local BBSes the low-rent district of the hacker culture, but
   they helped connect hackers and users in the personal-micro
   and let them exchange code.

   Use of this term for a Usenet newsgroup generally marks one
   either as a newbie fresh in from the BBS world or as a real
   old-timer predating Usenet.

   (2005-09-20)