The Collaborative International Dictionary of English v.0.48:

nonhierarchic \nonhierarchic\ nonhierarchical
\nonhierarchical\adj.
   Not arranged in a hierarchy. Opposite of hierarchical.
   [Narrower terms: ungraded, unordered, unranked]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

nonhierarchical
    adj 1: not classified hierarchically [syn: nonhierarchical,
           nonhierarchic] [ant: hierarchal, hierarchic,
           hierarchical]