An audience built from people similar to your existing customers, used for ad targeting.
Lookalikes (on Meta, TikTok, Google) are generated by an algorithm: you give it a seed list (your customers, or high-value segments within them), and it finds people who look similar based on behavior, interests, and demographics. Lookalikes usually outperform interest-based targeting because they're built from signal, not guesses.
Lookalikes scale what's already working. The best use: feed them your most-valuable customers (not all customers, your HIGH-LTV ones), and scale against that specific segment. The quality of your seed determines the quality of your lookalike.