Articulus (ar-tic’-u-lus): Roughly equivalent to “phrase” in English, except that the emphasis is on joining several phrases (or words) successively without any conjunctions (in which case articulus is simply synonymous with the Greek term asyndeton). See also brachylogia.
Articulus is also best understood in terms of differing speeds of style that depend upon the length of the elements of a sentence. The Ad Herennium author contrasts the the slower speed of concatenated membra (see membrum) to the quicker speed possible via articulus.
Ivanka. Donald. Donald Jr. Melania. Tiffany. Barron. Eric.
Ivanka is lovely and greedy.
Donald is lost.
Donald Jr. needs a few more visits to the orthodontist.
Melania wants a divorce.
Tiffany is a mystery.
Barron looks tired and malnourished.
Eric is terminally clueless.
Definition and commentary courtesy of “Silva Rhetoricae” (rhetoric.byu.edu).