so far is an adverb meaning "used in negative statement to describe a situation that has existed up to this point or up to the present time". It carries 3 recorded senses.
to the degree or extent that
“insofar as it can be ascertained, the horse lung is comparable to that of man”
“so far as it is reasonably practical he should practice restraint”
Definitions, synonyms and semantic relations for so far come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.