A Reinforcing Loop is a structure which feeds on itself to produce growth or decline

As State 1 increases or decreases State 2 is influenced to move in the same direction. State 2 then influences State 1 to continue to move in the same direction it is moving.

Because this structure is reinforcing it generally produces an exponential growth or decline. This exponential change may be unnoticeable for a period of time until it reaches the a noticeable range. The structure then seems to change very rapidly causing one wonders how it began all at once, when in fact it really didn't.