RankBrain is the nickname given to the proprietary artificial intelligence (or AI) used by Google to handle search queries. The AI may never be given a formal name, or maybe RankBrain will stick, but for now, it’s the name being used for the artificial intelligence that’s handling 15% of Google’s search volume.
An important distinction to be aware of is that RankBrain is not (currently) a standalone technology at Google – it’s incorporated directly into the existing algorithm that Google uses to rank web pages. However, while there are literally hundreds of ranking signals (and nobody outside Google knows for sure how many there are), RankBrain has quickly become one of the most important. Greg Corrado, a senior research scientist at Google, told the Washington Post that RankBrain is now the third most-important factor in determining how a search query will be answered on the SERPs.