Third-party data is personal or behavioural data collected by an entity with no direct relationship to the individual, then sold or licensed for targeting, lookalikes, or B2B enrichment. Common sources include data brokers like Acxiom, Experian, and LiveRamp. It contrasts with first-party data (collected directly from your visitors) and zero-party data (explicitly shared by the user). The category has collapsed in 2024-2026 — Chrome's cookie deprecation, Apple ATT, and GDPR consent pushed brands toward first-party strategies and data clean rooms like Snowflake and InfoSum. Under GDPR, third-party data still needs a lawful basis; under CCPA, sharing it usually triggers opt-out rights.