Learn in the textbookThe Framework of Machine Learning
DefinitionUnsupervised learning (教師なし学習) is a method that trains on data alone, without correct answers, and finds the groupings and structure hidden in that data.
ExplanationThe typical method is clustering (クラスタリング), which automatically puts similar data into the same group. It suits situations where no answer is set in advance, such as dividing customers into several segments by their buying habits. What each resulting group means is something people interpret by looking at what is in it.
AnalogyPicture a market you are visiting for the first time, where you sort vegetables you cannot name into baskets by similar shape and color. Nobody tells you the names, yet similar ones still end up together.
Memory hookThe Japanese name literally means "learning without a teacher": no answer labels, like being handed only the question sheet.