"A structure for storing data in a specific order; that is, an index that has data pages at the leaf layer. The main idea behind clusters is that all rows should be kept in order permanently according to some column value, such as the primary key." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)
"The process of storing data (records) near each other on disk because they have the same or similar key values, to obtain maximum query efficiency. Clustering applies to indexes as well, to coordinate the fast access to clustered data on disk." (Sam Lightstone et al, "Physical Database Design: The Database Professional’s Guide to Exploiting Indexes, Views, Storage, and More", 2007)
"Physically storing foreign key rows close to the primary key rows they reference to improve database performance." (Jan L Harrington, "Relational Database Design and Implementation" 3rd Ed., 2009)
"The process of partitioning the data attributes of an entity or table into subsets or clusters of similar attributes, based on subject matter or characteristic (domain)." (DAMA International, "The DAMA Dictionary of Data Management", 2011)
"Placing one record occurrence on the same database page as another record occurrence so that the physical I/O to access one occurrence will also access the other occurrence. A common clustering strategy involves the parent-child binary relationship; the child record type occurrences are placed physically near the parent record type occurrence so that there is a significant chance that the physical I/O to access the parent will also access the child." (George Tillmann, "Usage-Driven Database Design: From Logical Data Modeling through Physical Schmea Definition", 2017)
"Identifying groups of similar instances in a data set." (John D Kelleher & Brendan Tierney, "Data science", 2018)
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