A playing card was a type of card that is marked with symbols that belonged to a deck. A variety of card games could be placed with them. There were fifty-two cards to a standard deck.
While typically rectangular, playing cards might also come in a circular shape. (TOS: "Mudd's Women"; TNG: "The Measure Of A Man"; VOY: "State of Flux")
In the final draft script of "Mudd's Women", circular cards that appear in that entry were described as "an odd-sized deck of cards."
Fictional knights based on playing cards existed in the Earth classic Alice Through the Looking Glass. (TAS: "Once Upon a Planet")
Jake Sisko and Nog played a card game in 2369 while Nog was listening to a conversation between his uncle Quark and the waiter Broik in Quark's. (DS9: "Progress")
In 2376, when Harry Kim was navigating through a Borg cube, he used playing cards to mark his path, in case he had to come back the same way. He placed a three of hearts on a transwarp conduit and a nine of diamonds on a data node. (VOY: "Collective")
The term "trump card" has been used in behind-the-scenes sources, but not in canon.