Upside Down

Upside Down is a gameplay term.

Details
It refers to the orientation of card which is contrary position in which the card name appearing on the top of the card while it is facing you.

Currently, the only cards that can be put upside down in the battle zone are Fields (specifically: D2 Fields).

After their cost is paid, D2 Fields are put into the battle zone and remain there until removed by card removal, or another D2 Field is put into the battle zone.

They also feature an ability known as Denjara Switch.

At either the start of end of turn, you may put one of your D2 Fields upside down in order to trigger an ability. There is no way to change them back after they are put upside down without playing a new D2 Field.

Trivia

 * While cards put into your mana zone are placed in the opposite direction facing your opponent when they are untapped, it has nothing to do with this term of "Upside Down".