The process or action of grouping things together into clusters; in computing, a method of organizing data into groups.
From 'cluster' + gerund suffix '-ing'. The term expanded into computing and statistics during the 20th century.
In machine learning, clustering is how computers find patterns without being told what to look for—like how your brain recognizes that apples, oranges, and bananas are all 'fruits' without someone explaining it. It's unsupervised learning at work.
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