a) By losing to any opponent that fights him at the weight class he holds the title under.
b) By moving to a different weight class and not intending to defend his title.
c) By not defending his title for a period of more than 3 years.
d) Retirement.
e) Death.
NOTE: If he loses to a fighter at a higher or, catch-weight, then he can only lose the title if the fight was supposed to be held at said weight of the title and/or, if said opponent normally fights at the weight the title is held under.
Case in point; When Gomi fought Nick Diaz, the contest was at 160lbs. But Diaz had never fought at 155lbs. Since the fight was overturned, it is a moot point but if it had been upheld there still would have been no title change.
© Shane McFarland 2009