In games with playing cards, the one-eyed jack is the jack of hearts or the jack of spades. The male figure on the cards should be shown in profile exposing only one eye.
The most common place you'll hear this term is in a home poker game. A typical usage: "Ok boys, it's Seven Card Stud and one-eyed jacks are wild."
All the face cards have proper names going back hundreds of years. The proper names for these cards: