Rather than define what an eephus pitch is, it is easier just to show you what it looks like.
Not many dudes throw the eephus pitch. Here are some guys who threw it back in the day (click on name to see video or info on pitch): Rip Sewell (founder), Pascual Perez, Steve Hamilton, Phil Niekro, Orlando Hernandez, Dave LaRoche, Bill Lee, Dave Stieb, Kazuhito Tadano, and Bob Tewksbury.
Here are some dudes who are currently in the majors who have been rumored to throw an eephus pitch from time to time: Mark Buehrle, Casey Fossum, Liván Hernández, Randy Johnson, Armando Galarraga, and Tim Wakefield.
So I guess what I'm trying to say is sure, Livan Hernandez is the pitcher who originally ranked dead last among those projected to throw at least 100 innings, but at least Met fans can look forward to his eephus pitch potential.