Organ is a group of tissues that perform similar functions. A tissue is an organizational level that lies between cells and organs. Organs are formed by the functional grouping of multiple tissues. Two or more organs working together to perform a specific body function form an organ system which is also called the biological system or body system. Complete answer: The pineal gland is a small, pea-shaped gland that is located in the brain. It produces and regulates some hormones, including melatonin which is best known for the role it plays in regulating sleep patterns or also called circadian rhythms. The pineal gland is located in the epithalamus, near the centre of the brain, between the two hemispheres, which is tucked in a groove where the two halves of the thalamus join. The pituitary gland, or hypophysis, is an endocrine gland, which is about the size of a pea. It is a protrusion off the bottom of the hypothalamus located at the base of the brain. Hence, it is not the smallest organ. The thyroid glands measure about sixteen millimeter long, four millimetres wide, and two millimeter thick in the adult normal feline patient, which shows that it is not the smallest organ. The thymus increases in size till puberty, following which it decreases in size in a process known as involution. This also means that the thymus is not the smallest organ. Adrenal glands are orange-coloured glands located on the top of both kidneys. Adrenal glands are about the size of a fortune cookie and are triangular in shape, sitting on top of the kidneys like hats. Therefore, it is also not the smallest organ. The pancreas is a gland, about six inches long, which is situated in the abdomen. It is shaped like a flat pear and is surrounded by the stomach, small intestine, liver, spleen and also the gallbladder. Hence it is not the smallest organ. The actual size of an ovary depends on both the woman's age and the hormonal status, they are approximately three to five cm in length during childbearing years and become smaller and then atrophic once menopause occurs. The testis is oval in shape. Most men have two testicles, which are known as testes. One of the testicles will be a different size than the other testicle. Therefore, the Pineal gland is the smallest organ in the body. Note: Pineal gland also plays a role in the regulation of female hormone levels, and it affects fertility and the menstrual cycle. Its shape resembles a pine cone hence the name. The pineal gland is one of the neuroendocrine secretory circumventricular organs in which the capillaries are mostly permeable to solutes in the blood.