singleton vs playing card

singleton and playing card both are nouns.

Word	Noun	Adjective	Verb	Adverb
singleton	Yes	No	No	No
playing card	Yes	No	No	No

As nouns, playing card is a hypernym of singleton; that is, playing card is a word with a broader meaning than singleton:

singleton: the playing card that is the only card in a suit held in a bridge hand as initially dealt
playing card: one of a pack of cards that are used to play card games

singleton (noun)	playing card (noun)
the playing card that is the only card in a suit held in a bridge hand as initially dealt	one of a pack of cards that are used to play card games
a set containing a single member
a single object (as distinguished from a pair)