A natural phenomenon of the earth's atmosphere where imbalances in the static electric potential between parts of the atmosphere and/or the ground are removed or reduced through an electrical discharge.
Detailed description at:
http://thunder.msfc.nasa.gov/primer/index.html
In the field of software a system which enables C code to dynamically create machine code at runtime, aka RTCG. Under the GPL.
This package can be found at:
http://www.gnu.org/software/lightning/lightning.html