Cardinality
How many distinct values a column or dataset has — high cardinality (many unique) versus low (few).
Cardinality is the number of distinct values in a column or set. A user_id column has high cardinality (nearly all unique); a boolean or country column has low cardinality (few distinct values). It drives decisions across databases, indexing, and observability.
Worked example: a database index helps most on a high-cardinality column (email) where it narrows to few rows, and little on a low-cardinality one (a yes/no flag) where it still matches half the table; in metrics, high-cardinality tags (per-user) explode storage cost. Gotcha: high cardinality is powerful but expensive — it makes indexes selective and metrics detailed, but also blows up index size, partition counts, and time-series storage, so “add a per-request id tag” can quietly 100x your metrics bill.