This term was introduced by Aristotle in his Sophistic Refutations and inherited by medieval rhetoricians to indicate an ambiguous proposition in which no single term is equivocal (i.e., in which no single term can be interpreted as ambiguous).
For example, take the phrase "The killing of tyrants is justified". It can be interpreted to mean that tyrants' killing is justified, or that it is justifiable to kill tyrants. The proposition (i.e., phrase) is ambiguous, though each individual term (killing, tyrants, justification) remain clear and unambiguous.
Gritchka adds two classic examples from linguistics: "flying planes can be dangerous" and "the shooting of the hunters".