Adjective has 1 sense

• farthest(s = adj.all) farthermost, furthermost, furthest, utmost, uttermost - (comparatives of `far') most remote in space or time or order; "don't go beyond the farthermost (or furthermost) tree"; "explored the furthest reaches of space"; "had traveled to the farthest frontier"; "the utmost tip of the peninsula"

Adverbial has 2 senses

• farthest(r = adv.all) furthest - to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense); "chose the farthest seat from the door"; "he swam the furthest"; "see who could jump the farthest"
• farthest(r = adv.all) furthest - to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense); "furthest removed from reality"; "she goes farthest in helping us"; "went the furthest of all the children in her education"