Double down is a blackjack term and is the name of one of the decisions that a player can make as the game goes on. In most variants of blackjack, the player is allowed to double his initial bet in exchange for being forced to stand right after receiving one more card.
In land-based casinos, the additional bet is placed in the betting box right next to the original bet. Doubling down is rarely an advantageous move, but keep in mind that sometimes it might be the right thing to do. Please note that this phrase may also colloquially refer to any act of doubling your bet in a game like poker or even roulette.
“The dealer’s up card was 6 and he was supposed to hit on soft 17. I had a 3 and a 6 for exactly 9 points, so I immediately doubled down and won some additional cash thanks to following basic blackjack strategy.”
In a brick and mortar casino, players who wish to double down should place the additional chips beside their original bet, outside the betting box, and point with one finger. The dealer will then proceed to move your chips inside the box to indicate that your wager has been accepted and accounted for.