**Clustering** is the task of grouping a set of objects in such a way that similar objects lie within the same group (called *cluster*). These groups are not pre-defined beforehand, so it is an [[Unsupervised Machine Learning|unsupervised task]]. This "similarity" notion is vague enough to empower different uses. Popular notions of it include [[Distance Function|distance-based]] clusters, dense areas of data space, intervals or statistical distributions.