RankBrain is a machine-learning component of Google's core ranking algorithm, introduced in 2015, that helps interpret ambiguous or never-seen-before queries by mapping them to related concepts. Before RankBrain, unusual queries were handled by hand-crafted rules; RankBrain learns query-to-concept relationships from data, making Google better at understanding intent rather than matching exact keywords. It was one of Google's first AI-driven ranking signals and paved the way for later models like BERT and MUM. RankBrain is one of the top three ranking signals Google has publicly confirmed.