To surround something completely or to shut something in within boundaries. It can also mean to include something along with a letter or package.
From Old French 'enclos', past participle of 'enclore', from Latin 'includere' meaning 'to shut in'. The prefix 'en-' means 'in' and 'close' comes from Latin 'claudere' (to shut).
The word has a fascinating historical connection to land rights - 'enclosure' refers to the historical process where common lands were privatized and fenced off, fundamentally changing agricultural society. This same root gives us 'clause' in legal documents, as clauses 'shut in' or define specific terms.
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