A Lookalike audience is an audience similar to your existing customers/consumers/brand lovers (Custom audience).

“If you know who your best customers are now, Meta can help you find similar audiences domestically, in another country, or even internationally. Meta calls these lists of similar customers "lookalike audiences". You can create lookalike audiences in a country/countries of your choice, a region, a trade area (European Economic Area, North American Free Trade Agreement), in-app store countries or worldwide.”
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__ From Meta Help Center