What is hysteresis?

I recently wrote an article on the situation in Greece, and mentioned the effects of hysteresis which I will expand upon in this blog article.

Hysteresis is a theory developed by the Keynesians to explain why laissez-faire economic policy may be damaging in the long run. Neoclassicalists would argue that during an economic downturn, when an external shock causes demand to fall, wages should be allowed to fall which would increase the international competitiveness of the economy so that exports can grow to increase demand and provide a boost to the economy fueling further growth until the economy is out of the slump and growing again. [...]