How many of a thing there are, basically.
“Cardinality” refers to the number of elements in a set or other grouping. It’s a term commonly used in Data Science to refer to number-of-instances constraints between entities sharing a given relationship in an Entity-Relationship Diagrams. In mathematics, cardinality refers to the number of elements in a bound set. The set “1, 2, 3, 4, 5, 99” has a cardinality of 6. Note “Cardinality” is not ”Arity “.
Not sure what this is from anymore… but a related graphic:
Source
define cardinality - Google Search