cardinality

Everyday

1. (maths) The number of things in a set or group. tree →
2. (maths, counting) The idea that the last number you say when counting tells you how many there are in all. tree →

In the lessons

A maths word: the cardinality of the set {red, blue, green} is 3. In early counting lessons, the «cardinality principle» means the last number counted is the total. Phrases: «the cardinality of a set», «cardinality principle».

Examples

• The cardinality of the set {2, 4, 6, 8} is 4. tree →
• Count the apples: 1, 2, 3, 4, 5 — the cardinality is 5. tree →
• Two sets with the same cardinality have the same number of members. tree →
• The empty set has cardinality 0. tree →
• What is the cardinality of the set of days in a week? It is 7. tree →

Note

Technical maths word; in everyday talk say «how many». Related: «cardinal number» (one, two, three — how many) vs «ordinal number» (first, second, third — position). Repeated members count once in a set: {a, a, b} has cardinality 2.