Hormone Replacement Therapy (HET) refers to the medical use of hormones to treat symptoms that can be attributed to a relative or absolute deficiency of one or more hormones.
In the narrower sense, hormone replacement therapy refers to the administration of menopausal medications (menopause and virile men) and as a concomitant gender reassignment measure in the case of transsexuality.
The term itself is contentious because it is a subtype of hormone therapy and not a complete replacement of the endogenous hormone metabolism takes place.
Source: Wikipedia