Isolation levels
Varying strength of guarantees for how transactions see each other's data — Read Uncommitted, Read Committed, Repeatable Read, Serializable.
Lower level = faster but buggy. Anomalies: dirty read, non-repeatable read, phantom read, write skew. Postgres default: Read Committed. Many modern ORMs default to Repeatable Read. Serializable = ACID, but often too expensive.