Webster's English Dictionary

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sub

1. sub \'s*b\ aj : SUBORDINATE, SECONDARY {a ~ theme in a musical comp 
   osition}
2. sub n : SUBSTITUTE 
3. sub vb or subbed;  or sub.bing : to act as a substitute  : to apply a 
   substratum to (as a photographic film) 
4. sub n : SUBMARINE 
5. sub n [short for substratum] : a photographic substratum