"Fanon" is a combination of "fan" and "canon" and usually refers 
to a widely accepted fan assumption about the universe or 
characters of a specific show that did not appear in the canon 
but has been so widely disseminated in fandom that it becomes 
generally accepted.  Many fans assume that Starsky's father (who 
was killed when he was young) was a cop; likewise, many fans 
assume that Hutch comes from a well-to-do family that was 
emotionally cold.  Neither of these issues was ever brought up on 
the show, but so many fans have written about these two issues 
that they have become "fanon."

From Virginia Call:

Has anyone else here seen the use of the word '"fanon"?  I saw it 
used in The Sentinel groups and don't know if it is commonly  
used in other fandoms or not.

I've seen the word "fanon" to describe those little bits and 
pieces of fanfic that fans sometimes *confuse* as having come 
from the series itself. (It usually seems to be something 
popular, so more than one author will tend to use it.)  For a 
long time there was one thing that I thought actually came from 
and S&H ep, and that was that Starsky's father was a cop before 
he got killed.  It wasn't until later that I realized they never 
say what his father did for a living, and after reading it in 
fanfic assumed it was actually stated as such in the series.  (My 
Swiss cheese memory is famous for doing things like that! :  )

So Starsky driving a Ford Torino is 'canon' and Starsky's dad 
being a cop is definitely 'fanon'.
