An in-word is a word whose use indicates that the speaker is part of an exclusive group. Outsiders betray their otherness by failing to use that word when insiders would.
For example, businessmen who refer to Apple Corporation's Macintosh computer as "an Apple" instantly mark themselves as outside of the Macintosh user community. This community always calls the computer the "Macintosh" or a "Mac". Moreover, they reserve the term Apple for other Apple Corporation products such as the Apple II computer (and related models) or for the company itself.
Some in-words are code phrases which directly assert or deny group membership. Carnies speak of someone being "with it" or not, i.e., part of the carnival performer community (used in Stranger in a Strange Land). Some are used only to assert membership. A typical usage among American blacks is to refer to fellow blacks as "brother" or "sister" but not to apply these terms to whites or other ethnic groups.