Nidus: In medicine, any structure that resembles a nest in appearance or function. From the Latin for 'nest.' A nidus is a breeding place where bacteria, parasites, and other agents of a disease lodge and develop. For example, a nidus of infection is a focus of infection. A nidus is also the nucleus or origin of a nerve. The nidus avis cerebelli is a deep sulcus (groove) on each side of the inferior vermis (a wormlike structure in the brain), separating it from the adjacent lobes of the cerebral hemispheres.