One who encompasses; something that encloses, surrounds, or includes.
From encompass + -er (agent noun suffix). Creates a noun describing either a person or thing that performs the action of encompassing or surrounding.
This is a rare nominalization—you could call Earth an 'encompasser of continents' or the sky an 'encompasser of stars,' but we rarely use it because 'container' and 'enclosure' work better. English speakers prefer simpler words when metaphorical ones get too literary.
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