A sudden loss of lift at/near the stall, caused by a breakdown in attached flow over an area of the wing large enough and happening fast enough to register as a discontinuous jump on the 'g' trace if plotted against time.
One alternative would be a slow and/or localised flow breakdown, which might result in a mathematical CLmax being attained, but without the pronounced indication of the g'break.
If you were to plot a CL-versus-alpha plot, an aircraft with a g-break is more likely to have a very sharp spike at CLmax and a marked drop in CL just above that, whereas a non-g-break aircraft may have a more 'rounded' lift-curve.