1. In the kidney, a tiny ball-shaped structure composed of capillary blood vessels actively involved in the filtration of the blood to form urine: The glomerulus is one of the key structures that make up the nephron, the functional unit of the kidney.
2. In the olfactory bulb, an anatomically discrete module receiving input from an olfactory sensory neuron: Glomeruli are composed of receptor neutron axons and mitral neurons and arranged or formed by the type of odor .
3. One of the structures which comprise the nephron (functional unit) in the kidney: The glomerulus is composed of capillary blood vessels which are actively involved in the filtration of the blood.

The glomerulus in the kidney was so named by the Italian anatomist Marcello Malpighi (1628-1694). The structure was once called a "malphigian corpuscle".
4. Etymology: Glomerulus is the diminutive of the Latin glomus meaning "ball of yarn". It is literally a "little ball of yarn".