Give me your best jargon.

I’m compiling a list of jargon that represent fundamental ideas about different fields. You have any? Please give the word – short definition and where it is used. For example:

Hamiltonian is the operator corresponding to the total energy of the system – Quantum Mechanics.

Leader is the piece of intermediate light material that attaches a fly line to the fly. -Fly Fishing

and

Reciprocity is the correlation between aperture and shutter speed that results in the same exposure. – Photography