Felix Felicis (from the mock Latin for "happy happy") is a potion mentioned in the Harry Potter series of books, specifically the sixth book. The potion gives the drinker great fortune for twelve hours, being in effect "liquid luck". The potion can not be used too often because of its side effects, as well as the fact that it is expensive and tricky to make, and users are banned from competitive activities while under its influence.
The potion is important in the plot of the sixth book. Harry wins a vial of the potion from the potion's master, Horace Slughorn, and later uses its luck inducement to complete a difficult task; as well as giving some of it to his friends so that they survive the book's climactic battle. Much like the time turner from the third book, it is such an easy plot device that it is barred from usage in the seventh book, with the explanation that it takes too long to prepare.
Externally to the story, it is interesting with all the controversy that occasionally surrounded the series, there has not seemed to be too much comment on the nature of Felix Felicis, which is basically euphoria in a bottle. From the way the effects are defined, it seems like it would resemble anyone's given favorite substance, with the added benefits that the invincibility it gives is actual, and not imagined. Not that I imagine too many otherwise naive children have been turned down the path to addiction by a fictional potion, but it is, when considered, fairly suggestive.