Which gland in the human body is considered the "master gland"?

The pituitary gland.

The pituitary gland is about the size of a pea and is located at the base of the brain. Hormones produced in the pituitary often affect the function of other glands in the body, like the adrenals and thyroid, which are stimulated or inhibited based on the pituitary.

Control of the pituitary is mostly down to the hypothalamus, an area of the limbic system in the brain located very close to the pituitary, and connected to it by the infundibulum. The hypothalamus sends messages to the pituitary, controlling its productions, and by extension controlling the output of the other glands in the body.