Transparency
Transparency is a property of a question or clue which can buzzed on inappropriately early. A question with this property is transparent. 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.
Transparency is one of the hallmarks of bad question writing.
Theoretical Definition
Transparency has historically been used to describe questions whose clues are simply misordered or anti-pyramidal; for instance, if a giveaway clue appears in the middle of the question. Egregious examples of this are significantly less common now and are typically regarded as merely poor question construction rather than explicit transparency.
Transparency is now typically taken to mean lead-ins and other clues in power (or equivalent places in the question) which allow players to apply logic and reasoning to narrow the possible answer space to a small number of answers. This typically involves some level of knowledge of the canon.
Practical Definition
Transparent questions are difficulty to identify from statistics and are usually identified by players who notice issues with the question. Players who notice a transparent clue may be stymied from buzzing out of a belief that the question is "too easy".
In the older understanding of the term, it is possible to make fairly objective statements about whether a tossup was transparent after the fact because the relative difficulty of most clues is known. In the modern usage, it can be much more subjective - the degree to which a clue is "transparent" depends significantly on the domain knowledge of the player.
Examples
Certain pronouns have significantly reduced answer space, especially at lower difficulties and in certain subjects. For instance, a science question intended for high schoolers that uses the pronoun "this woman" has only a small handful of possible answers due to the constraints of the difficulty and is likely on Marie Curie, the most prominent female scientist. This is often circumvented by either choice of a new pronoun (e.g. "this person") or choice of a new answerline (e.g. a question on a specific result of Curie's or a field of study).
In a similar vein, tossups in specific subfields may be very transparent because of the small number of potential answers. For this reason, tossups on Grimm's Law or the Great Vowel Shift are almost always transparent.
A question's susceptibility to linguistics fraud is often considered a type of transparency - original language terms may make clues significantly easier for certain answerlines. The ability of players to deduce answers from what language a word in a clue "sounds like" is notoriously overestimated by many writers. Essentially no players at the middle school or novice high school levels, and sparingly few players at the regular high school or high school national levels, actually make buzzes based on this strategy with any regularity, and most questions below the college level would be better-served by using straightforward names and terms from foreign languages rather than using unnatural linguistic contortions to avoid them.
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.