  Bulk magnetic order in two dimensional La4Ni3O8 nickelate with Ni1+/Ni2+
(d9/d8), isoelectronic with superconducting cuprates is demonstrated
experimentally and theoretically. Magnetization, specific heat and 139La NMR
evidence a transition at 105 K to an antiferromagnetic state. Theoretical
calculations by DFT relate the transition to a nesting instability of the Fermi
surface with ordering wave-vector Q = [1/3, 1/3, 0].
