Definition of "Furthest" :

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

"Explored the furthest reaches of space."

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

"He swam the furthest."

adverb: 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."