Overenchanting is enchanting a piece of equipment beyond the relatively safe +x for each rank. Overenchanting will cause your weapon to glow. Moreover, stat increases gained from overenchanting is far more than that gained in the safe +1 to +x range.
Supposedly, stat increases gained from enchanting increase every x successes. i.e. the +(x+1) to +(2x) increases are greater than that of the +1 to +x; if the +1 to +x range gave you +1 P. Def per success, the +(x+1) to +(2x) range will give you more than +1 P.
Def and the +(2x+1) to +(3x) range will give you more than the +(x+1) to +(2x) in terms of P. our +4 Belt gives me +4 P. Def overall as a result of enchanting. When we successfully overenchant it to +5, it will give us something around +6 P. Def and not +10 P. Def.