1. To defeat a selection mechanism by introducing a (sometimes temporary) device that forces a specific result. The word is used in several industries; telephone engineers refer to spiking a relay by inserting a pin to hold the relay in either the closed or open state, and railroaders refer to spiking a track switch so that it cannot be moved. In programming environments it normally refers to a temporary change, usually for testing purposes (as opposed to a permanent change, which would be called hardwired).
2. [borderline techspeak] A visible peak in an otherwise rather constant graph (e.g. a sudden surge in line voltage, an unexpected short “high” on a logical line in a circuit). Hackers frequently use this for a sudden short increase in some quantity such as system load or network traffic.