Obscure, dark, vague, enigmatic, cryptic, ambiguous, equivocal are applied to language or expression (or less often to causes, motives, or thoughts) with the meaning not sufficiently intelligible or clearly understood.
Something is obscure the true meaning of which is hidden or veiled, because of some fault or defect either in the thing itself or in the person trying to understand it.
Something is dark which is imperfectly revealed and therefore mysterious.
Something is vague which is lacking in distinct outlines or in clear definition, either because it is too general or because it is so imperfectly conceived or thought out that it is incapable of clear formulation.
Something is enigmatic which puzzles, mystifies, and, often, baffles one seeking its true meaning or significance.
Something is cryptic which is stated or expressed darkly or enigmatically; the word often implies a definite intention to perplex or to challenge.
Something is ambiguous which admits of more than one interpretation, largely because of the use of words having a dual or multiple meaning without giving an indication of which sense is intended.
Something is equivocal which permits a wrong or false impression, thereby admitting uncertainty and confusion or fostering error. As applied to use of words, equivocal is distinguishable from ambiguous in that it may suggest intent to deceive or evade.
In extended use equivocal is applied to something such as an act or a mode of life that admits of two possible or plausible interpretations, one of which may be harmful or discreditable.