Window Card
A window card is a card dealt face up. In stud, it means an upcard, and the first of them is called the door card. In Hold'em and Omaha, the window card is the first card of the flop that the dealer turns over.
The simple explanation
In seven-card stud, some of your cards are face up so everybody can see them. They are window cards. In Hold'em, when the dealer puts out the flop, the first card visible as it is placed is called the window card.
A closer look
The window card has no special rules. It is slang that comes from the idea of a window into the hand: you see a part of it before the whole. The first upcard in stud, the door card, gives information about a player's hand and affects the betting order, since the lowest door card has to bring in. In Hold'em, players use the term in stories: 'there was an ace in the window'.
A practical example
In a stud hand, a player's up card is the king of spades. That is his door card and window card.
Full explanation
A window card is a card dealt face up. In stud, it means an upcard, and the first of them is called the door card. In Hold’em and Omaha, the window card is the first card of the flop that the dealer turns over.
A window card is a card that is dealt face up.
In stud, the first one is the door card.
In Hold’em, it is the first card of the flop that is seen.
It is slang, not a rule.