I have to disagree with the definition: "A recommendation engine is a feature (not a product) that filters items by predicting how a user might rate them." The

I have to disagree with the definition: "A recommendation engine is a feature (not a product) that filters items by predicting how a user might rate them." The feature bit is correct, but the rating bit might not be. People very often give high ratings to things they are not interested in. People may highly rate jewelry that they cannot afford, or give a five star rating to a movies considered classics, that they don't actually want to view. What you are trying to optimize is ultimately purchasing, continued subscription, ad views, or some other profit metric. That can be hard to directly measure, so something like ratings can be a stand in, but you may accidentally optimize for something deeply undesirable like recommending expensive jewelry to everyone that they will never buy.