Operational amplifier, so called because it allows mathematical operations.
The generic opamp has two inputs and one output. The voltage at one of the inputs is inverted, summed with the voltage at the other input, and and the sum is present at the output.
outout = non_inverted + inverted
If the output is connected to the inverting input, after being scaled by a voltage divider, then
output = (-K * output) + input
output = 1/(1+K) * input
We have an amplifier.