Definition of Portsmouth
1a port and naval base on the southern coast of England; population 201,800 (est. 2009). The naval dockyard was established here in 1496. 2a historic port city in southeastern New Hampshire, on the Piscataqua River, off the Atlantic Ocean; population 20,443 (est. 2008). 3a commercial and naval city in southeastern Virginia, on Hampton Roads, west of Norfolk; population 100,577 (est. 2008).