WordNet (r) 3.0 (2006):

boil down
    v 1: be the essential element; "The proposal boils down to a
         compromise" [syn: reduce, come down, boil down]
    2: be cooked until very little liquid is left; "The sauce should
       reduce to one cup" [syn: boil down, reduce, decoct,
       concentrate]
    3: cook until very little liquid is left; "The cook reduced the
       sauce by boiling it for a long time" [syn: reduce, boil
       down, concentrate]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "boil down":
   abbreviate, abridge, abstract, bob, capsulize, clip, compress,
   condense, contract, crop, curtail, cut, cut back, cut down,
   cut off short, cut short, dock, elide, epitomize, foreshorten, mow,
   nip, poll, pollard, prune, reap, recap, recapitulate, reduce,
   retrench, shave, shear, shorten, snub, streamline, stunt, sum up,
   summarize, synopsize, take in, telescope, trim, truncate