K3 lattice
“second integral cohomology group of a K3 surface, endowed with the bilinear symmetric form given by cup product, is an even unimodular lattice of signature (3,19) and is unique up to isomorphism. Thus (see also §3.1 below for the notation), there is an isometric 2 32 isomorphism of H (X;Z) with the lattice A = H ©Eft . We call A the K3 lattice.”