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A simple definition of an easement of support may be the right of an owner of a building not to have the support which he receives from the land or buildings of his neighbour removed without replacement. Put more legalistically, an easement of support involves an enhancement to the dominant owner's natural right of support with a corresponding increase of the servient owner's obligations to refrain from interference. An easement of support may though also involve a diminution of the dominant owner's obligations to refrain from depriving the servient tenement of the support to which the servient owner would otherwise have been entitled.

http://www.merriam-webster.com/dictionary/right of support

Definition of RIGHT OF SUPPORT
1: the easement or servitude acquired by grant or by prescription by which an owner of a structure on land has a right to rest or support it in whole or in part upon the land or structure of an adjoining owner (as by inserting beams in the adjoining wall on the boundary)

http://definitions.uslegal.com/e/easement-of-support/

An easement of support is acquired either by prescription or by grant. The natural right of support applies primarily to support provided to the land by other land. Buildings, however, can acquire a right of support, known as an easement of support, from other land or from other buildings.