Before revision: Given a set S of n d-dimensional points  , the k-nearest neighbors ( KNN) is the problem of quickly finding k points in S that are nearest to a query point q.
After revision: In machine learning, classifiers are used to predict a class of a given query based on an existing (classified) database. Given a database S of n d-dimensional points , the k-nearest neighbors ( KNN) is the problem of quickly finding k points in S that are nearest to a query point q.