Proposed definition #2: What cannot be said
Introverted Intuition is the attitude of attunement to what cannot be said, by virtue of the structuring that "saying" requires.
For example: At work, we don't dare say our true feelings (or we can only say them if they're positive), because we know that sharing them would bring dire economic consequences. There is no other way, because the structure of the workplace (people working cooperatively to get stuff done that they get paid for) requires that people refrain from saying anything that might put their loyalty in doubt. If an accountant, in his office, says that he loves accounting, you view this as meaningless because, well, what else is he going to say? In fact, he might very well hate accounting. You have to be highly attuned to what's really going on in order to read the true meaning of what people say--which is often the opposite of the literal meaning of their words. (See Eric Berne.)