Also called or "Deficiency-evil" refers to when abnormal Qi from the four seasons attacks the body when it's weak.

Defensive Qi

Is when one of the six abnormal changes of the climate, i.e. wind, cold, summer heat, dampness, dryness or fire attack the body, usually without being noticed also called theft-wind because it's like a thief sneaking into someone's house unnoticed.

The Qi of Winter is Yin, cold and still in nature. Related to the element water and associated with the emotions fear and the organs kidneys and bladder.