definition. diagram, shape [leinster2016basic, 5.1.18] [tt-0025]

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Let \({\cal C}\) be a category and \({\cal J}\) a small category. A functor \(\mathscr {D} : {\cal J} \to {\cal C}\) is called a diagram in \({\cal C}\) of shape \({\cal J}\).

\({\cal J}\) is also called the indexing category of the diagram, and we say that \(\mathscr {D}\) is a diagram indexed by \({\cal J}\) [rosiak2022sheaf, example 35]. \(J\) is also called the template.