Endometrial Thinning refers to the reduction in the Thickness of the endometrial lining, which can be a desired effect in the treatment of certain gynecological conditions or an Unintended consequence of hormonal imbalances. It is often achieved through the Use of medications like progestins or GnRH Agonists and is important for procedures like In Vitro Fertilization where a Thinner lining may be beneficial.