Normally taken to be that part of London south of the River Thames
According to the 2011 official Boundary Commission for England definition, London 2011 amendment), the area includes the London boroughs of Bexley, Bromley, Croydon, Greenwich, Kingston, Lambeth, Lewisham, Merton, Richmond, Southwark, Sutton and Wandsworth.
Wikipedia page [1] refers.