The Collaborative International Dictionary of English v.0.48:

Groundwork \Ground"work`\ (ground"w[^u]rk`), n.
   That which forms the foundation or support of anything; the
   basis; the essential or fundamental part; first principle;
   as, development of a convenient DNA sequencing technique
   layed the groundwork for many of the subsequent advances in
   molecular genetics. --Dryden.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

groundwork
    n 1: the fundamental assumptions from which something is begun
         or developed or calculated or explained; "the whole
         argument rested on a basis of conjecture" [syn: basis,
         base, foundation, fundament, groundwork,
         cornerstone]
    2: lowest support of a structure; "it was built on a base of
       solid rock"; "he stood at the foot of the tower" [syn:
       foundation, base, fundament, foot, groundwork,
       substructure, understructure]
    3: preliminary preparation as a basis or foundation; "we are
       prepared today because of groundwork that was done ten years
       ago"

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "groundwork":
   arrangement, base, basement, basic training, basis, bearing wall,
   bed, bedding, bedrock, bottom, briefing, clearing the decks,
   cornerstone, equipment, familiarization, fixing, floor, flooring,
   fond, footing, foundation, fundament, fundamental, ground, grounds,
   hardpan, infrastructure, makeready, making ready, manufacture,
   mobilization, pavement, planning, prearrangement, preliminaries,
   preliminary, preliminary act, preliminary step, prep, preparation,
   preparatory study, preparing, prepping, prerequisite, pretreatment,
   principle, processing, propaedeutic, provision, radical, readying,
   riprap, rock bottom, root, rudiment, seat, sill, solid ground,
   solid rock, spadework, stereobate, stylobate, substratum,
   substruction, substructure, terra firma, training, treatment,
   trial, tryout, underbuilding, undercarriage, undergirding,
   underpinning, underpinnings, understruction, understructure,
   warm-up