Ruff (cards)

In trick-taking games, to ruff means to play a trump card to a trick (other than when trumps were led). According to the rules of most games, a player must have no cards left in the suit led in order to ruff. Since the other players are constrained to follow suit if they can, even a low trump can win a trick. In some games, like Pinochle and Preferans, the player who cannot follow suit is required to ruff. In others, like Bridge and Whist, he may instead discard (play any card in any other suit). Normally, ruffing will win a trick. But it is also possible that a subsequent player will overruff (play a higher trump). This is not always a bad thing—see uppercut below.

"Ruff" is normally a verb, meaning "to play a trump card when a non-trump suit was led". "To trump" can be used as a synonym of "to ruff", but "ruff" is normally preferred, for clarity. As a noun, "ruff" and "trump" are completely different -- "a ruff" means only "an instance of ruffing", while "(a) trump" means only "the suit that outranks all other suits", or "a card in this suit". Hence:

W             E

West plays the grand slam of 7♠ despite having only 7 high card points. The declarer can draw the outstanding trump king, ruff the diamonds in dummy, going back to the hand by club ruffs. Unless both minor suits are divided 6-0, one of the minor suits will ultimately become high and provide the missing two tricks. In summary, the declarer took one trick by leading a high card (the ace of trumps) and 10 tricks by cross-ruffing; the remaining two tricks came as result of long suit establishment.

N

W               E

S

A ruff and discard (also known as ruff and slough or ruff and sluff) occurs when a player leads a suit that neither opponent has - typically in a suit contract, a defender leads a suit in which dummy and declarer are both void and dummy and declarer have at least one trump each. This gives declarer the option of discarding a losing card from one hand while playing a trump from the other, usually garnering an additional trick in the process. Thus, the ruff and discard is generally to be avoided by the defenders, except in rare cases where declarer has no side suit loser to discard. It is often inflicted upon the defence via an endplay.

...
Wikipedia