Lead In

1. A Lead In is a mini story which occurs before the main plot. It is used to warm up the audience and set up the events which lead to the main story. Aside from being a catalyst, it usually has very little relevance to the rest of the episode. It may also be used to stretch the script to a sufficient length.

A Lead In differs from The Teaser in that:
 * It does not occur before the credits (and is therefore not intended to grab the audience before a channel change).
 * The Teaser does not have to be a separate set up, it can be an intrinsic part of the main plot.

A lead-in and a teaser are similar in that they both serve to bring the audience into the story.

Live Action TV

 * Six Feet Under begins with a Lead In showing a person's death. The remainder of the show is sometimes related to that person, and sometimes not.
 * MacGyver employed a variation on this in several episodes. An extended pre-credits sequence called the "Opening Gambit" was used several times, perhaps as a time-filler. It would be a short, self-contained adventure with an entirely different writer, production team, and supporting cast, and was often radically different in style from the rest of the episode (The titular character hardly ever employed any Bamboo Technology in the Opening Gambit), and featured its own credits. After the gambit, the titles would roll, and the story proper would begin. The only link between the gambit and the main story of the episode would be a voice over to the tune of "No sooner had I gotten back from that mission when they sent me on this one," designed to convey nothing more than that MacGyver leads a very action-packed life.
 * Mystery Science Theater 3000's Lead In is usually its own self-contained comedy sketch, unrelated to the rest of the episode except for brief reference after the commercials.
 * Sliders, mostly in early seasons, always started with the main characters in a random world ready to slide into the world that would be the setting for the episode. These lead-in stories rarely contribute to the main adventure itself (with some exceptions, like "The Breeder")
 * Law and Order explores the activities of the people who discover the body in the minute or so before it is discovered.

Literature

 * Books 1, 4, 6, and 7 of Harry Potter start with such a chapter. In fact, the sixth double-stacks.
 * But they all are part of the main plot, just elsewhere.

Newspaper Comics

 * Many newspaper comic strips do this for Sunday installments. The first two panels will feature a small gag which is loosely related to the main one. This is done because the top row of a Sunday strip (which usually consists of two panels and the title) are often cut out by newspapers, so the main content of the strip can't begin before the second row of panels without alienating some readers.
 * Calvin and Hobbes famously averted this in its later years; once the strip became popular enough that he had some clout, creator Bill Waterson insisted that newspapers either run the Sunday strip unchanged, or not run it at all.

Western Animation
"Abe: Aw, I can't believe we went through all that just to wind up with a tennis court.
 * Family Guy uses this in almost every episode.
 * The Simpsons have used a lead in with almost every episode as an intentionally convoluted path to the main story. It is so convoluted at times, the writers feel compelled to use a Call Back or blatant Lampshade Hanging referencing the Lead In.
 * A Tale of Two Springfields [BABF20] -- Badgers invade the family doghouse and Homer discovers that Springfield has been split into two area codes upon calling animal control. The split is the source of conflict for the rest of the plot. At the very end of the episode, we get a Call Back with waves of long forgotten badgers invading Springfield.
 * This led to a very blatant lampshading when, in the middle of a scene dealing with the area code split, the badger appears at the window, growling menacingly, before Homer tells it to "Go away, we got bigger problems now."
 * Tennis the Menace [CABF07] -- There's a notably long Lead In in which Homer helps Abe find a final resting place. During shopping around, the funeral director mentions that a very extravagant monument "will consume as much space as a regulation-size tennis court." The scene changes and Homer has used his money to build a tennis court rather than a tomb. Just as the viewer realizes how convoluted things became, we experience this Lampshade Hanging.

Homer: I'll bet you didn't see that coming."

- 2. As in the expression "lead in audience".
 * The Simpsons has gotten very bad with this lately, to the point that the Lead Ins can take up more than half the episode!
 * Ed, Edd 'n' Eddy uses this every episode.

A lot of TV viewers will watch a programme, then decide to watch the thing that is on next.

Therefore, having the right show before you can make a huge difference in your Ratings- often over a million viewers (Lost in the UK shed 1.3 million viewers between premiere and its next showing; Big Brother wasn't on for the second week).

Stations compete to get good lead-ins for their local news broadcasts. At 5 or 6, it's usually a syndicated show, at 11 it's usually a network show. So, the NBC affiliate would often win the 11 o'clock ratings battle back when ER was on top at 10, and whoever gets Oprah will win at 5.

This is important, since all the local news broadcasts are more or less interchangeable in the mind of the viewer, and it's also usually the only ad revenue the station doesn't have to share with the network or syndicate. This was one of the reasons The Jay Leno Show, which was demonstrably hurting the 10/11 p.m. newscasts of NBC affiliates, was cancelled.

Examples:

 * Given that live eviction shows would be a highlights show and The Reveal, with a half-hour break before the exit interview, Big Brother, as mentioned above, was the example of this for Channel 4, supporting every series of The Friday Night Project and Eight Out Of Ten Cats. Until the later series, where it began to need a Lead In of its own.