narrow down
     v 1: define clearly; "I cannot narrow down the rules for this
          game" [syn: {pin down}, {peg down}, {nail down}, {narrow},
           {specify}]
     2: define clearly; "Can you pin down the difficulties" [syn: {pin
        down}]
     3: become more special; "We specialize in dried flowers" [syn:
        {specialize}, {specialise}, {narrow}] [ant: {diversify}, {diversify}]
Source: WordNet® 2.0