A piece of paper, cardboard or an index card used to contain information for a mechanic in a LARP. The cards themselves do not actually exist in game, and you may not trade,steal, destroy or otherwise alter the cards without using a mechanic. When you see someone playing a card in a game, you see them holding out a piece of paper, but you character does not see their character holding the paper, instead, your character sees their's doing what is described on the card, often nothing.
Generally there is one distict card for each ability, skill, effect or other mechanic that the character has. Ability cards are most often used for things which interact with other player's characters, such as "you are now my Brainwashed Loyal Agent". They are also used for other mechanics, and as reminders to the players they are given to of detail of mechanics, or specific character skills or information.
When printed on both sides, one side generally has the requerments for using the ability, and the other side has the effect on the target. The target is only shown the effect side, as the requierment side often contains details of the mechanic that the target does not need to know, helping to prevent metagaming. In cases where the card does not affect other characters, the effect side may contain flavor text which has no actual game effect.