Full Idea
Ambiguity results from the possibility of transforming different underlying truth-conditional structures into the same surface form.
Gist of Idea
Ambiguity is when different underlying truth-conditional structures have the same surface form
Source
Gilbert Harman (Thought [1973], 5.3)
Book Reference
Harman,Gilbert: 'Thought' [Princeton 1977], p.78
A Reaction
Personally I would call a 'truth-conditional structure' a 'proposition', and leave it to the philosophers to decide what a proposition is.