Window
A window is a card that is exposed or flashed at the end of a player's closed hand. It is the card that you can see by accident.
The simple explanation
If a player holds his cards loosely and one of them is visible, that is a window. Other players can see it, so it gives away information.
A closer look
In stud games, 'window card' means an upcard. In other situations, a window is a mistake, and may be a deliberate act called window dressing. Players should protect their cards to prevent this. If you notice that an opponent has exposed a card, it is better to tell the dealer than to use the information.
A practical example
A player holds his cards in a fan, and a corner of one card is visible to the player next to him. That is a window.
Full explanation
A window is a card that is exposed or flashed at the end of a player’s closed hand. It is the card that you can see by accident.
A window is a card exposed at the end of a closed hand.
It gives information.
Window dressing is the deliberate version.
Protect your cards.