Component with an impedance that has a negative real part
Not with passives, so far as I know, but there are amplifiers you can build that have effectively negative input impedance, and they generally use positive feedback to achieve this.  For example, see http://en.wikipedia.org/wiki/Negative_impedance_converter

From the pointed-to wikipedia entry:



This example shows resistances, but R3, for example, can certainly be a capacitor.  The situation I've generally seen these used in are to cancel out the large impedances of glass microelectrodes that are pulled from pipettes, used to record neural signals