adjective
very great in degree
[more ~; most ~] :very serious or severe
[more ~; most ~] :very far from agreeing with the opinions of most people :not moderate
always used before a noun :in the farthest possible position
sports
unusual and dangerous
b always used before a noun US :involved in an unusually dangerous sport :competing in an extreme sport
go to extreme lengths
noun
plural extremes
[count] either one of two opposite conditions, feelings, positions, etc., that are thought of as being far from what is normal or reasonable
an amount or degree that is far beyond what is normal or reasonable
If you carry/take (something) to extremes or go to extremes, you do much more than most people would consider reasonable or normal.
in the extreme
formal :to the greatest possible degree - used to make a statement more forceful