1. A nest or place in which birds, insects, spiders, etc. deposit their eggs: During their stroll on the beach, Janine and James discovered two nidi of sandpipers, or small wading birds, that usually pick up insects, worms, and soft mollusks in the mud and sand near oceans.
2. A breeding place in a living organism where disease germs develop: Nidus is a focus of origin; such as, a collection of bacteria of infections in a living organism.
3. A point or place at which something starts, develops, or accumulates; for example, the center around which salts of calcium, uric acid, or bile acid form calculi: The nidus is the beginning of a crystal or similar solid deposit.