Slash fiction (also known as "m/m slash" or slashfic) is a genre of fan fiction that focuses on romantic or sexual relationships between fictional characters of the same sex. While the term "slash" originally referred only to stories in which male characters are involved in an explicit sexual relationship as a primary plot element, it is now also used to refer to any fan story containing a romantic pairing between same-sex characters. Many fans distinguish slash with female characters as a separate genre, commonly referred to as femslash (also known as "f/f slash" or "femmeslash").
These fan-written stories are not accepted canon, and the characters are usually not engaged in such relationships in their respective fictional universes.
It is commonly believed that slash fan fiction originated during the late 1970s, within the Star Trek: The Original Series fan fiction fandom, starting with "Kirk/Spock" stories generally authored by female fans of the series.
The name arises from the use of the slash symbol (/) in mentions in the late '70s of K/S (meaning stories where Kirk and Spock had a romantic [and often sexual] relationship), as compared to the ampersand (&) conventionally used for K&S or Kirk and Spock friendship fiction. For a time, both slash and K/S (for "Kirk/Spock") were used interchangeably. Slash later spread to other fan groups, first Starsky and Hutch, Blake's 7, and The Professionals, then many others, eventually creating a fandom based on the concept of slash. Many early slash stories were based on a pairing of two close friends, a "hero dyad", or "One True Pairing", such as Kirk/Spock or Starsky/Hutch; conversely, a classic pairing between foils was that of Blake/Avon from Blake's 7.
The first K/S stories were not immediately accepted by all Star Trek fans. Later, authors such as Joanna Russ studied and reviewed the phenomenon in essays and gave the genre some academic respectability.