A μητρόπολις is a 'mother-city' in relation to her colonies—Athens to the Ionians, Doris to the Peloponnesian Dorians, Thera to great cities founded from her, Corinth to Corcyra. The term captures the colonial relationship where one city founds another and retains a parental status. Metaphorically, it can mean 'source' or 'origin,' as when the brain is called the 'metropolis of the cold' or geometry is deemed the 'beginning and mother-city' of other sciences. Later usage extends it to mean any principal city.