The Collaborative International Dictionary of English v.0.48:

ill-formed \ill-formed\ adj.
   1. (Grammar) not grammatical; ungrammatical; not conforming
      to the rules of grammar or accepted usage. Opposite of
      grammatical.

   Syn: ungrammatical.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

ill-formed
    adj 1: not grammatical; not conforming to the rules of grammar
           or accepted usage [syn: ungrammatical, ill-formed]
           [ant: grammatical, well-formed]