farthest

1. To or at the most distant or remote point.
2. To or at the most advanced point or stage.
3. By the greatest extent or degree.

[Middle English ferthest, superlative of farther. See farther.]

adjective

Most distant or remote, as from a center: extreme, farthermost, furthermost, furthest, outermost, outmost, ultimate, utmost, uttermost. See big/small/amount, edge/center.

Definition: most distant
Antonyms: closest, nearest

The adjective has 2 meanings:

Meaning #1: most distant or remote

Meaning #2: (comparatives of `far') most remote in space or time or order
  Synonyms: farthermost, furthermost, furthest, utmost, uttermost

The adverb farthest has 2 meanings:

Meaning #1: to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense)
  Synonym: furthest

Meaning #2: to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense)
  Synonym: furthest