Anti-possession tattoo

Anti-possession symbols are a method to ward off demonic possession. It features a pentagram, or endless knot, which is the reason why it protects those who wear this symbol.

Usage
In the series, follwoing Sam's possession by Meg in "Born Under A Bad Sign", Bobby gives both Sam and Dean each a Charm Amulet which features a symbol which will stop demonic possession.

Then, the boys reveal that they have had these symbols tattoed on their chests.

Origin

Although Dean was originally going to have a tattoo - as seen in this behind the scenes in Phantom Traveler - it was in Season 3 that both Sam and Dean got tattoos.

Sera Gamble aswered about the tattoos origins in an interview:

'''FNN: Those matching tattoos are pretty cool. How long has that been planned?'''

SG: I remember talking about it with Ben Edlund last season, while Cat was writing "Born Under A Bad Sign." We agreed that if we were Sam, the first thing we'd do is take that anti-possession amulet and head for the nearest tattoo parlor. He reminded me of it while we were working on "Jus In Bello," because breaking out the amulets was an important story point in that episode. So I stuck it in there, and it made for a nice moment. It's like, of ourse Sam and Dean have matching tats. How could it be any other way? By the way, I'll be convinced we're truly a cult hit when a fan gets the same tattoo.

Trivia

 * Dean's tattoo is seen again in the opening scene of "Lazarus Rising".
 * In "The Monster At The End Of This Book", the phenomenon of fans getting tattooed because of the cult to the series is referred to when Sam and Dean reveal their tattoos in oreder to pass as avid fans. In response, the publisher of the "Supernatural" books by Chuck Shirley, reveals she has the same tattoo - on her ass. While not named in the episode, the publisher's name was Sera Seige, named after writers Julie Siege and Sera Gamble - as mentioned in this interview by Sera Gamble.