From ill + defined.

ill-defined

1. Poorly defined; blurry, out of focus; lacking a clear boundary.
   • 1837, L[etitia] E[lizabeth] L[andon], Ethel Churchill: Or, The Two Brides. […], volume III, London: Henry Colburn, […], →OCLC ↗, pages 45–46 ↗:

     An ill-defined dislike, a little like jealousy, arose in Henrietta's mind; a little, however, mitigated by observing that the gentleman instantly caught sight of herself; and that, when not absolutely forced to look at his companion, he looked towards her.

2. (mathematics) Defined in an inconsistent way.

• (poorly defined) fuzzy, hazy; see also Thesaurus:indistinct
• (defined in an inconsistent way)

• well-defined