What is circuit input voltage?

Some articles on circuit, voltage, input voltage, input:

Implementation - Op-amp Implementations - Non-inverting Schmitt Trigger
... In this circuit, the two resistors R1 and R2 form a parallel voltage summer ... It adds a part of the output voltage to the input voltage thus "helping" it during and after switching that occurs when the resulting voltage is near the ground ... The output of the parallel voltage summer is single-ended (it produces voltage in respect to ground) so, the circuit does not need an amplifier with a differential input ...

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