Definitions of releasing hormone:

• noun:   any of several hormones produced in the hypothalamus and carried by a vein to the anterior pituitary gland where they stimulate the release of anterior pituitary hormones; each releasing hormone causes the anterior pituitary to secrete a specific hormone
• noun:   a substance produced by the hypothalamus that is capable of accelerating the secretion of a given hormone by the anterior pituitary gland