Transparency is one of the hallmarks of bad question writing. It is generally defined as a mismatch between the distribution of points where the question is answered and the distribution of places where the question should be answered.
A tossup is transparent if logic and reasoning, stock clues, or inappropriate pyramidality narrow the possible answer space to one answer before uniquely identifying, "hard" clues appear. Occasionally, transparency can be cited if a giveaway clue appears in the middle of the question; however, transparency is usually reserved for lead-in clues.
Transparency is difficult to determine from statistics, because one of the following four cases will occur during any transparent tossup:
1. A buzzer race before the power mark or equivalent place in the question. Both teams complain about the tossup.
2. One team buzzes early, while the other team does not know what just happened. The team that got the tossup complains about it. This typically happens when the team that does not get the tossup is composed of novices who are still learning mid-level and giveaway clues and have not been around long enough to recognize stock clues.
3. One team buzzes early, while the other team stares in disbelief that the question could be "that easy". Both teams complain about the tossup.
4. Both teams wait for more information, unable to believe that the question could be "that easy". A buzzer race ensues on a later clue. Upon revelation of the answer, both teams complain about the tossup.
Since none of these cases can be inferred from available data except in rare cases, practical definitions of transparency often hinge on post-tournament anecdotes.
Tossups about Grimm's Law or the Great Vowel Shift are almost always transparent.
Things that are not "transparent"
The fact that some team, somewhere, knew a clue does not make a tossup "transparent." In fact, well-designed tossups should be answered early by good teams. Overuse of the term "transparent" to complain about questions being powerable by good teams has made the word nearly useless in recent years and will probably get you mocked if you try to use it non-ironically in a quizbowl discussion.