The Collaborative International Dictionary of English v.0.48:

Laconism \Lac"o*nism\, n. [Gr. ?, fr. ? to imitate Lacedaemonian
   manners, to speak laconically: cf. F. laconisme.]
   1. A vigorous, brief manner of expression; laconic style.
      [1913 Webster]

   2. An instance of laconic style or expression.
      [1913 Webster]

WordNet (r) 3.0 (2006):

laconism
    n 1: terseness of expression [syn: laconism, laconicism]