The flood of vampires and vampire related (and to a lesser extent, Stephanie-Meyer-related) themes and stories into American pop culture in the late 2000's due to the adaptation of the vampire novels in the "Twilight" series into film form, where women of all ages fell in love with pale, melodramatic people who sparkle in sunlight whilst not being able to act to save their lives.
The effects include, but are not limited to, various merchandise, such as action figures, documentaries, and games with the Twilight license, the dramatic increase in women of all ages at Comic-Con, The Vampire Diaries, Tru Blood, Stephanie Meyer and her adaptations getting green-lit, various vampire-themed comic books by Stephen King and Marvel
Comic Book Geek #1: Dude, the X-Men are going to be fighting vampires now? What the frak?
Comic Book Geek #2: Twilight Effect, indeed.
"Twilight Effect" is when something is given too much hype and credit. The object subject to the "Twilight Effect" is raved about to the point that the amount of praise actually reduces its credibility as well as its quality.
Person1: ONE DIRECTION, MAN.
Person2: WHAT ABOUT IT MAN.
Person1: THEY'RE TOTALLY THIS GENERATION'S BEATLES, MAN.
Person2: MAN, THAT SUCKS. I'M SORT OF DISGUSTED AT HOW PEOPLE THINK THAT. THIS REDUCES ITS CREDIBILITY AS WELL AS ITS QUALITY.
Person1: OH I GET YOU NOW MAN. TWILIGHT EFFECT, MAN.
Teenagers showing up in the ER complaining of severe bite wounds to the neck. Caused by a partner trying to be romantic and fit with the new vampire fetish spawned by the plot of Twilight. If the bite is not deep enough to draw copious amounts of blood, the bactieria in the partners mouth may cause a nasty infection that requires serious hospitalization.
Dude : Did you hear about the 30,000 bite wound cases reported in ERs this year? They call it the twilight effect, stupid people trying to act like vampires.
Chick: Dont judge them, vampires are hot.
Dude: Because nothing says "I love you" like losing a quart of blood.
Chick: You suck, your nothing like Edward Cullen, im going ot read twilight 100 times to purge my mind of any trace of reality.