Not defined clearly or distinctly: an ill-defined concept; an ill-defined view across the foggy moor. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "ill-defined" in a sentence
"But the term is ill-defined, so explain references to it in earnings articles and also include net income, the figure required by the SEC and accountants."
"In other words having a real-world, timely-measurable, good impact on a group of people for whom their problems and the relevance of technologies for solutions are open and ill-defined questions at the beginning of study, raises significant challenges and ones that seem at odds with the ways that Computer Science does disciplinary business."
"In the search for answers, there is no reason to stop at M-theory, which is not strictly speaking a theory at all but an ill-defined speculation about how string theory, itself a speculation about quantum gravity, may cohere."