The process by which electrical voltage and/or current (signal) levels are reduced by combining them with otherwise identical signals that are shifted or reversed in phase, is called "bucking".
Humbucking is a process in which "hum" that is causing objectionable artifacts, generally in audio or video systems, is reduced or eliminated (bucked) in electrical and/or electronic systems and/or devices. This is a term used when the reduction is accomplished through the use of devices that introduce a 180 degree phase shift only in the undesirable portion of the signal in such a way that, when the modified signal is combined with the original, the unwanted part of the signal is reduced to a point where it is no longer noticeable and the desired part of the signal is not noticeably affected.
Some common applications of this process are:
Humbuckers (Humbucking guitar pickups).
Humbucking microphones (the Sennheiser MD421, Shure SM63 and others).
Humbucking transformers or coils used in video systems.
Telephone (and other audio) system and computer communications wiring.