The term door or door card is a term used to describe a player's first upcard in a poker hand.
In games where individual players have upcards of their own, their initial upcard is called their "door card". In games where players do not have individual upcards but instead share a flop of community cards, the door is the first card visible of the flop. Players will often talk about a particular card being "in the door" of the flop, as in:
"I had pocket kings, she had pocket queens. We were both all-in before the flop. I was feeling good until I saw a Queen. Right in the door!"