Lead-in
A leadin or lead-in is the first clue of a tossup. Per the rules of pyramidality, it is the most difficult clue to buzz off of in a tossup. Yet in pyramidal quizbowl, since leadins must abide by all of the rules for good clues, the leadin will inform the player of what specifically is being asked for (i.e. a title, an element, a concept, etc.) and will consist of a clue or clues that uniquely identify the answerline. Thus when writing leadins, writers should take especial care to avoid stock clues and vague clues.
Leadin etiquette
Leadins often contain information that is more tangentially related to the subject at hand. Examples include literary criticism about a novel (rather than descriptions of the plot or characters), opinions of historians about a given historical event (rather than specific facts describing said event), or literature about works of art (rather than descriptions of the work of art itself).
Pop culture in leadins
Though tangential academic information is generally acceptable in leadins to academic tossups, pop culture clues should generally be avoided for leadins unless the question itself is a mixed pop culture/academic question.
Bonus leadins
The term "leadin" can sometimes also refer to the introduction to a bonus, before the first part proper. ("Intro(duction)" for bonuses may be preferred to avoid ambiguity.) Bonus introductions typically identify the theme of the bonus or state an interesting or little-known fact about the first answer, though care should be taken to avoid writing overly lengthy (e.g. anything over 2 lines usually) leadins. In an oft-criticized practice, some writers insert (usually terrible) jokes or other funn content into their bonus leadins.