Definition of mass verb from the Oxford Advanced American Dictionary

     

    mass

     verb
    verb
    NAmE//mæs//
     
    [intransitive, transitive]Verb Forms present simple I / you / we / they mass
     
    he / she / it masses
     
    past simple massed
     
    -ing form massing
     
     
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  1. to come together in large numbers; to gather people or things together in large numbers (+ adv./prep.) Demonstrators had massed outside the embassy. Dark clouds massed on the horizon. mass somebody/something The general massed his troops for a final attack.
  2.  
    adjective the massed ranks of his political opponents
See the Oxford Advanced Learner's Dictionary entry: mass