The Collaborative International Dictionary of English v.0.48:

Plinth \Plinth\, n. [L. plinthus, Gr. ? a brick or tile, a
   plinth, perh. akin to E. flint: cf. F. plinthe.] (Arch.)
   In classical architecture, a vertically faced member
   immediately below the circular base of a column; also, the
   lowest member of a pedestal; hence, in general, the lowest
   member of a base; a sub-base; a block upon which the moldings
   of an architrave or trim are stopped at the bottom. See
   Illust. of Column.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plinth
    n 1: an architectural support or base (as for a column or
         statue) [syn: pedestal, plinth, footstall]