Nonlinearity
Etymology
Built from Latin roots: non ("not") plus linearis ("of or belonging to a line," from linea, "a line, thread"). A "linear" relationship plots as a straight line, with output rising in fixed step with input. Nonlinearity names every relationship that departs from that straight line — where small changes can trigger disproportionate results.
noun · /non-lin-ee-AIR-ih-tee/
The property of a system whose output is not proportional to its input, so effects can be far larger or smaller than their causes.
What it names
Why doubling the effort sometimes does nothing and sometimes changes everything — the reason cause and effect so often refuse to keep a steady ratio.
Example
Sleep is brutally nonlinear: six hours leaves you fine, but drop to four and your whole day falls apart.
Related idea
Nonlinearity is disproportion between cause and effect in general; a threshold is one sharp form of it, where the response stays flat and then jumps.