The pineal gland, also known as the 'pineal body,' is a small endocrine gland. It is located on the back portion of the third cerebral ventricle of the brain, which is a fluid-filled space in the brain. This gland lies in-between the two halves of the brain. In adults, it is approximately 5 to 9 mm long and 0.1 grams in weight. However, it is often larger before puberty. The gland is named for its shape, which resembles a pinecone (in Latin, 'pinea'). It is composed of neuroglial cells, portions of neurons, and cells known as 'pinealocytes.' Pinealocytes are types of endocrine cell.

The pineal gland's function in the body is not clearly understood yet. However, it is known to play a role in regulating female reproduction and sexual maturation. It also has a part in controlling circadian rhythms, the body’s internal clock that affects such actions as when we wake and sleep. The pinealocytes create and secrete melatonin, a hormone that helps maintain the body’s internal clock. Humans generally have higher levels of melatonin in childhood, which progressively decrease with age. Unusually high melatonin levels have been linked to a delay in sexual maturation. Melatonin also helps regulate female reproductive hormones, including when women menstruate.