WordWeb Online

1. (comparatives of 'far') most remote in space or time or order
   "don't go beyond the furthest tree"; "explored the furthest reaches of space"
   - farthermost, farthest, furthermost, utmost, uttermost

1. To the greatest degree or extent or most advanced stage ('furthest' is used more often than 'farthest' in this abstract sense)
   "went the furthest of all the children in her education"; "furthest removed from reality"
   - farthest
    
2. To the greatest distance in space or time ('farthest' is used more often than 'furthest' in this physical sense)
   "he swam the furthest"
   - farthest

See also: far