Cardinality refers to the number of elements in a set or the size of a collection. In mathematics, it helps to compare different sets by determining whether they have the same number of elements, more elements, or fewer elements. For example, the set 1, 2, 3 has a cardinality of 3, while the set a, b has a cardinality of 2. In the context of databases, cardinality describes the uniqueness of data values in a particular column. High cardinality means many unique values, like a column of Social Security Numbers, while low cardinality indicates fewer unique values, such as a column of Gender. Understanding cardinality is essential for efficient data organization and retrieval.