Definitions of furthest:

• adjective:   (comparatives of `far') most remote in space or time or order

  Example: "Explored the furthest reaches of space"

• adverb:   to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense)

  Example: "Went the furthest of all the children in her education"

• adverb:   to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense)

  Example: "He swam the furthest"