  One of the leading issues in high-$\rm T_c$ copper oxide superconductors is
the origin of the pseudogap phase in the underdoped regime of their phase
diagram. Using polarized neutron diffraction, a novel magnetic order has been
identified as an hidden order parameter of the pseudogap as the transition
temperature corresponds to what is expected for the pseudogap. The observed
magnetic order preserves translational symmetry as predicted for orbital
moments in the circulating current theory. Being now reported in three
different cuprates familles, it appears as a universal phenomenon whatever the
crystal structure (with single CuO$_2$ layer or bilayer per unit cell). To
date, it is the first direct evidence of an hidden order parameter
characterizing the pseudogap phase of high-$\rm T_c$ cuprates.
