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Any attribute that can have multiple values associated with a single instance of some entity. For example, a book might have multiple authors.
Such a "-to-many" relationship might be represented in an unnormalised relational database as multiple author columns in the book table or a single author(s) column containing a string which was a list of authors. Converting this to "first normal form" is the first step in database normalisation. Each author of the book would appear in a separate row along with the book's primary key. Later nomalisation stages would move the book-author relationship into a separate table to avoid repeating other book attibutes (e.g. title, publisher) for each author.