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