Slash is a genre of fanfiction which focuses on a romantic and/or sexual relationship (or a romantic and/or sexual relationship to be) between two characters of the same gender. The earliest stories in the genre, apparently, were Kirk/Spock stories, AKA "K/S". The genre was named for the "/" symbol between Kirk and Spock's initials, which would designate the story as... well, slash.

(I only say "apparently" because I find it really hard to believe that K/S was the first slash when Holmes and Watson had been around 50+ years prior. <g>)

If this really isn't your cup of tea, well, I'm not sure how you made it through my site so far, but here's an exit for you.

However, if you're curious...
For more on the history of slash as we know it, check out DocSci's Foresmutters Project. Resources/links are available at Fan Fiction On The Net, Slash Fiction Online, the Slash Page Database Project and the Slash section of The Fan Fiction Directory.