A trump is a playing card which is elevated above its normal rank in trick-taking games. Typically an entire suit is nominated as a trump suit - these cards then outrank all cards of plain (non-trump) suits. In other contexts, the term trump card can refer to any sort of action, authority, or policy which automatically prevails over all others.
The word "trump" derives from "trionfi" or "triumph", documented as the name of a card game in 1529 and which spawned the game Ruff and Honours, which in turn led to Whist. Trionfi was also the name of the original card game for which tarot cards were designed, and in it the tarot cards had the role of what are now called trumps; later card games, beginning with Triomphe, started using one of the ordinary suits as a replacement for the tarots when a tarot pack was not available.
The trump suit may be fixed as in Spades, rotate on a fixed schedule or depend on the outcome of the previous hand as in Ninety-nine, be determined by drawing a card at random as in Bezique, by the last card dealt to a designated player as in Whist, by the first card played as in Nine Card Don, be chosen by a designated player as in Barbu, or players may bid for the right to select the trump suit as in Contract Bridge or Skat.
In most games, trump cards cannot be played if the player has any cards of the suit led to the trick; the requirement to "follow suit" is of higher priority. In a few games, trumps can be played at any time. Playing the first trump to an already-started trick is known as trumping or ruffing; if another player were to play a higher trump, that would be an overruff or overtrump.
The tarot deck contains a fifth suit, known in gaming as the atouts or honours and in occult circles as the Major Arcana, which serves as a permanent trump suit in games played with the tarot deck. The suit consists of twenty-two cards, including a Fool which serves as a highest trump (in Central Europe) or excuses the players from following suit elsewhere.
Due primarily to the prevalence of the trump in card games, the term used in Japan for the standard 52-card deck of playing cards is simply the English word "trump".
From this card game use came metaphoric uses, e.g.:
In general strategy, a trump card is the move one party can use to attain decisive victory. In this sense, a trump card can be a person, weapon, or the starting of a chain of events.