Definition
At low power levels, the fundamental output power rises in a one-to-one ratio (in terms of dB) of the input power, while the second-order output power rises in a two-to-one ratio. If the input power is high enough for the device to reach saturation, the output power flattens out in both the first- and second-order cases.
The second order intercept point is the output power point at which the extrapolated first- and second-order lines intersect on a plot, since the actual power levels will flatten off due to saturation at much lower power level typically. In other words, the response is assumed to be perfect all the way to infinity. There are actually values for both the input and output SOI (known as the ISOI & OSOI) of a device or system, being related by the gain of the device or system. The OSOI in dB is simply the ISOI in dB plus the gain of the device or system.
Read more about this topic: Second-order Intercept Point
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