For a lattice gas it is <δnx(t)δnx(0)>, where nx(t) is the fluctuation of the number of particles in point x at time t. Its coarse grained version for an atomic monolayer adsorbed at a solid surface can be probed by STM. See the STM image below showing the diffusion of N adatoms on Fe(100) surface [M. Pedersen et al., Phys. Rev. Let. 84, 4898 (2000)].