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1. Arrange or order by classes or categories
   "How would you classify these pottery shards--are they prehistoric?";
   - class, sort, assort, sort out, separate
    
2. Declare unavailable, as for security reasons
   "Classify these documents"
    
3. Assign to a class or kind
   "How should algae be classified?";
   - relegate

Derived forms: classified, classifies, classifying

Type of: assign, attribute, categorise [Brit], categorize, control, restrict

Antonym: declassify

Encyclopedia: Classify