Overpressure is airpressure below normal or atmospheric pressure.

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Overpressure means pressure that is higher than the surrounding air pressure. The word “over” signals an increase above ambient, not a decrease. Atmospheric pressure at sea level is about 14.7 psi, and any pressure above that is considered overpressure (positive gauge pressure). If the pressure is below atmospheric, that’s underpressure (negative pressure or vacuum). So the statement is incorrect because overpressure is not below normal; it’s above it. A quick way to remember is: overpressure = more than ambient; underpressure = less than ambient.