Definition of as preposition from the Oxford Advanced Learner's Dictionary

      

    as

     preposition
    preposition
    BrE BrE//əz//
     
    ; NAmE NAmE//əz//
     
    ; BrE strong form BrE//æz//
     
    ; NAmE strong form NAmE//æz//
     
     
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  1. 1  used to describe somebody/something appearing to be somebody/something else They were all dressed as clowns. The bomb was disguised as a package.
  2. 2  used to describe the fact that somebody/something has a particular job or function She works as a courier. Treat me as a friend. I respect him as a doctor. You can use that glass as a vase. The news came as a shock. She had been there often as a child (= when she was a child).
  3. Word Origin Middle English: reduced form of Old English alswā ‘similarly’ (see also).
See the Oxford Advanced American Dictionary entry: as