A downward slope or the act of sloping downward, especially used to describe the side of a hill or mountain that goes down.
From Latin declivitas, formed from declivis 'sloping downward,' which combines de- 'down' and clivus 'hill or slope.' The word entered English in the 1600s as a technical term for topography.
The Romans had this word because they were obsessed with slopes—every road, aqueduct, and building required understanding how water would flow. Declivity is the opposite of acclivity, and knowing both words would have made you indispensable to Roman engineers.
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