Unsupervised Learning
Finding structure in data that has no labels — grouping, compressing, spotting outliers.
Unsupervised learning looks for patterns without being told what to look for. Typical jobs are clustering similar items, reducing dimensions, and flagging anomalies. There is no accuracy score in the usual sense, so evaluating the result takes judgement.
In practice: Segmenting customers into groups nobody defined in advance, based only on purchase behaviour.