Input resistance is an OK term. It effectively states that it's measured under a DC condition. In generic terms, one can call it input impedance. An scope may have 1 megohm shunted by 20 pf as an input impedance.
An easy experimental way to find it with an amplifier is to put a resistor in series with the input (initially set to zero). Now increase the resistor until the output is 1/2. Take out the resistor and measure. That's the input resistance of the amplifier.
Generally we will know by context.
You can say that my voltmeter has a 10 Megohm input impedance or input Resistance. Older analog meters had an input resistance in units of ohms/volt. Say 50K/volt. So, if you were measuring 2V, your meter would look like a 100 K resistor.
Obviously in the above circuit, you do not know the inductance and capacitance terms, but they should be negligible at the frequency of interest.
Impedance really starts to matter when they are transmission lines. Examples are 50 ohms, 75 ohms, 300 ohms. Cable mismatches in impedance cause reflections in the cable.
I'm not sure I confused you more or not. The important thing is to understand it as the teacher presents it and spit it out that way unless you can prove without a doubt that he is wrong. You tend to learn at the ability that you can comprehend. Later, you may have to totally throw out the concept or you find out the concept only works in that specific case.
e.g. Slope of a line is a special case of slope of a curve.