operational amplifier (thing)

The bastion of analog computing. "op-amp" is short for "operational amplifier."

An op-amp is a circuit whose output is the sign of the difference of the two voltage levels applied to the inputs.

Here's a schematic diagram:

          V+
         |
    | -  |
    |   -|
    |     -
----|+      -
    |xxx      >---Vout
----|-      -
    |     -
    |   -|
    | -  |
         |
         V-

As I said, a lone op-amp will only output the sign of the difference between the inputs. The sign is determined by subtracting the voltage level at the "-" input from the "+" input. If the sign is positive:

V_out = V+

If the sign is negative:

V_out = V-

If you want to use an op-amp to amplify by a certain factor, you need to use negative feedback.