Basix is a finite element definition and tabulation runtime library. Basix allows users to: @itemize @item evaluate finite element basis functions and their derivatives at a set of points; @item access geometric and topological information about reference cells; @item apply push forward and pull back operations to map data between a reference cell and a physical cell; @item permute and transform DOFs to allow higher-order elements to be use on arbitrary meshes; @item interpolate into and between finite element spaces. Basix includes a range of built-in elements, and also allows the user to define their own custom elements. This package provides the C++ library for Basix.