HPA axis:

HPA axis diagram showing the cascade of signals from the hypothalamus to the anterior pituitary to the adrenal glands. Image courtesy of Brian M Sweis.

short for hypothalamic-pituitary-adrenal axis. The HPA axis is a term used to represent the interaction between the hypothalamus, pituitary gland, and adrenal glands; it plays an important role in the stress response.