Trump
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.
Etymology
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.
Trump in card games
In most games, the relative rank of cards within a suit is the same in trump and plain suits, but they may sometimes differ, for example in Klabberjass, Euchre, or Eighty Points.