Kerodon

Construction 5.4.5.1. Let $\operatorname{\mathcal{C}}$ be an $(\infty ,2)$-category. We let $\operatorname{Pith}(\operatorname{\mathcal{C}}) \subseteq \operatorname{\mathcal{C}}$ denote the simplicial subset consisting of those simplices $\sigma : \Delta ^ n \rightarrow \operatorname{\mathcal{C}}$ which carry every $2$-simplex of $\Delta ^ n$ to a thin $2$-simplex of $\operatorname{\mathcal{C}}$. We will refer to $\operatorname{Pith}(\operatorname{\mathcal{C}})$ as the pith of $\operatorname{\mathcal{C}}$.